Hunter's Mark doesn't really rescue two weapon. Hunter's Mark is nice... for builds that don't use their bonus action for something else. The fact it's using the same resource as two weapon fighting means it costs you a substantial amount of damage to actually cast it, so you're actually behind if you spend less than 3 turns on a given target, and even at 3 turns you're only ahead by 5% or so, which is hardly worth the AC loss of not using a shield.
You can dual wield them with feat anyway, and light weapon property is not only about weight, it's about balance, it is long and peiercing making it less practical to use, you usually need to thrust it or swing it skillfuly to make only the tip hit the target, which requires much more skill, so it's use is kinda complicated, regular light weapons on the other hand is not so complicated) And yes the balance issue as well(though dual wielding rapiers or wielding a rapier with a dagger or short sword have both historical and modern references, and in truth might be practical especially for duels, that is why we have the dual wielding feat and fighting syle, i'd still prefer using a buckler with it though)
Hunter's Mark doesn't really rescue two weapon. Hunter's Mark is nice... for builds that don't use their bonus action for something else. The fact it's using the same resource as two weapon fighting means it costs you a substantial amount of damage to actually cast it, so you're actually behind if you spend less than 3 turns on a given target, and even at 3 turns you're only ahead by 5% or so, which is hardly worth the AC loss of not using a shield.
Well, hunter's mark is worth 7 DPR across two hits. So as long as the BA attack is comparable, the first round is functionally a wash. It's on subsequent rounds that the spell starts to make a real difference, adding a mean 10.5 DPR across three hits. It's not a bad investment once Extra Attack kicks in, typically from 5th-level on, but being in melee does make you more likely to take damage and lose concentration. And we are ignoring the out-of-combat application of hunter's mark. Namely, the advantage it grants to track the target.
Of course, the above math assumes the attacks always hit. Reality says otherwise, and I think the odds of landing a hit are supposed to be about 2/3. So let's bring those numbers down to 4.67 and 7, respectively. That said, weapon attacks will suffer similar drops, so we don't actually have to do this math. I just wanted to be thorough.
In practice, it's fine. It's possible to optimize for TWF and deal respectable damage while also buffing your AC slightly. Most people get blinded by the +10 damage from GWM and SS; forgetting that the -5 to hit means their effective damage increase is only +1 damage per attack. But those big numbers look cool, and that's what people fixate on. TWF doesn't look cool.
Well, hunter's mark is worth 7 DPR across two hits. So as long as the BA attack is comparable, the first round is functionally a wash.
You should be comparing to someone who isn't using 2WF.
In round 1, a Dueling ranger who uses Hunter's Mark and has 18 Dex does 2d8+2d6+12 (28), whereas the 2WF does 4d6+8 (22).
In round 2, the Dueling ranger does another 28 (total 56), the 2WF now does 33 (total 55)
In round 3, the Dueling ranger is up to 84, the 2WF is up to 88, so he pulled ahead...by 4 damage.
If we're going to do this, let's be thorough about it. Both builds are 5th-level; with a proficiency bonus of +3 and a +4 to their Ability modifier. This should account for Extra Attack, any necessary spellcasting, and their respective fighting style. We're not assuming race, so no feats; though a variant human or custom lineage could throw this off slightly. They're also up against a CR 5 enemy, which the DMG suggest should have an AC of 15. With no magic items, or other bonuses to hit, this means they need a 7 or better on the D20 to hit. So, their odds of failure are 30%.
Each attack from the duelist hits for 1d8+1d6+6; for a mean of 14. But with a 65% of a base hit and a 5% chance of a critical hit, this means their effective damage per hit is closer to 10.2, or 20.4 damage per round. And this will remain consistent, so adding it up over multiple rounds is easy enough.
The two-weapon fighter hits for slightly less, dealing 2d6+4 per hit; for a mean of 11. After applying the to-hit and crit chance, we're at 8.15 damage per hit, or 16.3 damage for the first round. But, as you've pointed out, this potentially goes up on subsequent rounds. Three attacks on round two means they'll deal an expected 24.45 damage that round.
That brings our two-round totals to 40.8 and 40.75. The difference is negligible. Over a protracted fight against a tough enough enemy, the TWF with hunter's mark will deal more. But, and this is the key thing to remember, it isn't all about the individual numbers. The team wins or loses together, and battle conditions can turn on a dime. It isn't a race, and these kinds of...measuring contests...don't serve any practical purpose.
Hunter's Mark doesn't really rescue two weapon. Hunter's Mark is nice... for builds that don't use their bonus action for something else. The fact it's using the same resource as two weapon fighting means it costs you a substantial amount of damage to actually cast it, so you're actually behind if you spend less than 3 turns on a given target, and even at 3 turns you're only ahead by 5% or so, which is hardly worth the AC loss of not using a shield.
You can dual wield them with feat anyway, and light weapon property is not only about weight, it's about balance, it is long and peiercing making it less practical to use, you usually need to thrust it or swing it skillfuly to make only the tip hit the target, which requires much more skill, so it's use is kinda complicated, regular light weapons on the other hand is not so complicated) And yes the balance issue as well(though dual wielding rapiers or wielding a rapier with a dagger or short sword have both historical and modern references, and in truth might be practical especially for duels, that is why we have the dual wielding feat and fighting syle, i'd still prefer using a buckler with it though)
Well, hunter's mark is worth 7 DPR across two hits. So as long as the BA attack is comparable, the first round is functionally a wash. It's on subsequent rounds that the spell starts to make a real difference, adding a mean 10.5 DPR across three hits. It's not a bad investment once Extra Attack kicks in, typically from 5th-level on, but being in melee does make you more likely to take damage and lose concentration. And we are ignoring the out-of-combat application of hunter's mark. Namely, the advantage it grants to track the target.
Of course, the above math assumes the attacks always hit. Reality says otherwise, and I think the odds of landing a hit are supposed to be about 2/3. So let's bring those numbers down to 4.67 and 7, respectively. That said, weapon attacks will suffer similar drops, so we don't actually have to do this math. I just wanted to be thorough.
In practice, it's fine. It's possible to optimize for TWF and deal respectable damage while also buffing your AC slightly. Most people get blinded by the +10 damage from GWM and SS; forgetting that the -5 to hit means their effective damage increase is only +1 damage per attack. But those big numbers look cool, and that's what people fixate on. TWF doesn't look cool.
You should be comparing to someone who isn't using 2WF.
If we're going to do this, let's be thorough about it. Both builds are 5th-level; with a proficiency bonus of +3 and a +4 to their Ability modifier. This should account for Extra Attack, any necessary spellcasting, and their respective fighting style. We're not assuming race, so no feats; though a variant human or custom lineage could throw this off slightly. They're also up against a CR 5 enemy, which the DMG suggest should have an AC of 15. With no magic items, or other bonuses to hit, this means they need a 7 or better on the D20 to hit. So, their odds of failure are 30%.
Each attack from the duelist hits for 1d8+1d6+6; for a mean of 14. But with a 65% of a base hit and a 5% chance of a critical hit, this means their effective damage per hit is closer to 10.2, or 20.4 damage per round. And this will remain consistent, so adding it up over multiple rounds is easy enough.
The two-weapon fighter hits for slightly less, dealing 2d6+4 per hit; for a mean of 11. After applying the to-hit and crit chance, we're at 8.15 damage per hit, or 16.3 damage for the first round. But, as you've pointed out, this potentially goes up on subsequent rounds. Three attacks on round two means they'll deal an expected 24.45 damage that round.
That brings our two-round totals to 40.8 and 40.75. The difference is negligible. Over a protracted fight against a tough enough enemy, the TWF with hunter's mark will deal more. But, and this is the key thing to remember, it isn't all about the individual numbers. The team wins or loses together, and battle conditions can turn on a dime. It isn't a race, and these kinds of...measuring contests...don't serve any practical purpose.