Ugh. Still just as useless of a spell as before. Why even include this spell at all if it requires another entirely different spell just to have any kind of use? At least change the casting time to 1 minute instead of an entire hour. Has anyone ever even cast this spell in the history of 5e?

This spell will probably be very useful for clerics since the changes to divine intervention allow them to cast this with an action by expending a channel divinity.  This feature also bypasses the material component of the spell (a jewel worth 1,000+ GP, which the spell consumes).

What summoning spell works with this? The 2024 they summon a spirit that is an ally 

It seems like it's meant to be more of a DM spell to explain how an enemy could mechanically bind a planar being to serve it. Still not great in the hands of PCs unless they use magic circle properly, in which case this spell lets you summon a powerful planar being, bind it to your service for a ridiculous amount of time, and have a quirky planar ally for an exceedingly long time.

Side note: I don't see how a creature that you have to bind to work to your service would ever NOT be hostile and twist your commands while serving you, because if they were a friendly celestial or something you don't need to magically enslave them and make a magic circle or anything in the first place.

This should be a ritual but reduced casting time.

Quote from Wrathamon >>

What summoning spell works with this? The 2024 they summon a spirit that is an ally 

Gate is the only one that comes to mind, but Planar Ally might also count depending on how the DM interprets it.

Quote from Abelhawk >>

Ugh. Still just as useless of a spell as before. Why even include this spell at all if it requires another entirely different spell just to have any kind of use? At least change the casting time to 1 minute instead of an entire hour. Has anyone ever even cast this spell in the history of 5e?

To me, this is one of those spells that really should be in the DMG rather than the PHB, because it's way better for NPCs. An archmage or lich can use this in the background to amass a small army of bound Elementals, Fiends, etc.

All summoning spells work with this. That's the point of "If the creature was summoned or created by another spell, that spell’s duration is extended to match the duration of this spell." And "Typically, the creature is first summoned into the center of the inverted version of the Magic Circle spell to trap it while this spell is cast." 

Summoning spells require concentration. So you can: 

Step 1: cast Magic Circle - inverted, so it acts like a trap. 
Step 2: Cast Summon xxxx and place the creature inside the circle so it is "trapped" there and you don't have to concentrate on it while you:
Step 3: Cast Planar Binding on the summoned creature, so that summoned creature's duration is " extended to match the duration of this spell." 

Now you've got a summoned creature that you don't have to maintain concentration on for 24 hours or more. You can also give it more complex commands as well. "accompany you on an adventure, to guard a location, or to deliver a message. "

I agree that the material component is way too expensive. 

A variable material component cost to determine length of the binding I think would work better. 

Quote from Abelhawk >>

Ugh. Still just as useless of a spell as before. Why even include this spell at all if it requires another entirely different spell just to have any kind of use? At least change the casting time to 1 minute instead of an entire hour. Has anyone ever even cast this spell in the history of 5e?

 
It just means you need to prepare. You can lay a trap with a Glyph of Warding that triggers an inverted magic circle, and another Glyph that triggers Planar Binding when you step on it. Then you just need to lure an outsider onto the first Glyph.

Quote from labmonkey2k >>

A variable material component cost to determine length of the binding I think would work better. 

 
That's how Planar Ally works. Binding is for forcing outsiders to obey you.

And they still didn't say what to do with concentration when extending duration 

Cleric is slave master now

I don’t understand the point to putting this on the Druid spell list. Without a spell like Magic Circle, how is a Druid supposed to force a powerful Fiend, Elemental, etc. to hold still for an hour?

Answering my own question here: looks like Charm Monster or Antipathy Sympathy could conceivably be used, at least on targets/creature types that aren’t immune to being Charmed.

Can I use this on anyone? Could you theoretically use this on an Arch Fay or something? 

Most summoning spells only last an hour or so. If a friendly Celestial you summoned wanted to help out it would have to leave after 1 hour. This way it can stay forever assuming you Recast the spell every time it's about to expire.

Using high level spell slots you can summon an army of powerful creatures in downtime. Obviously costs a lot of money but it's still a cool spell. Alternatively you can just use it to keep a summon with you forever (for example cast summon Celestial at level 6 and then use this spell at level 5, long rest, then use it again at level 6 so you have it with you for 10 days without concentration). Very interesting spell.