Mage Armor

Using a shield adds to your AC because the spell says "the target’s base AC". In the rules glossary, your base AC is equal to 10 + your Dexterity modifier, and a shield just adds a bonus to your AC if your proficient with it.

You don't don a shield.  You hold it.  So it is most definitely allowed RAW.  Wording matters.  

what always pissed me off is that mage armor actually uses dex not int modifier making it kinda meh in my opinion unless u max dex wich u seem a bit forced to do 

You can use this spell to lower enemies AC's, particularly dragons who have a significantly lower DEX score compared to their natural armor, lower their AC by 4 or something for half the day with no concentration or save, and bewilder your DM!

Quote from KingKronosTheTimeGod >>

You can use this spell to lower enemies AC's, particularly dragons who have a significantly lower DEX score compared to their natural armor, lower their AC by 4 or something for half the day with no concentration or save, and bewilder your DM!

The spell explicitly says it can only be used on a willing creature.

and that is what charisma is for, it is still really useful for this purpose if you only use a little bit of social manipulation. 

It's also worth pointing out that the rules say that if you have more than one way to calculate your base AC, you get to choose which one to use at any given time. This would mean that you'd not only need to convince the enemy to let you cast this spell on them, but also convince them that they should be choosing to have a lower AC when you attack them.

Yes, but my main point with the original post was that this spell specifically says "Until the spell ends, the target’s base AC becomes 13 plus its Dexterity modifier.", It sets the AC.

Also those rules you are referring to are specifically in the Multiclassing, class features section and specifically references the Monk's Unarmored defense and the Sorcerer’s Draconic Resilience feature which both say "your base Armor Class equals" compared to this spells "the target’s base AC becomes".

This section also starts out with,

When you gain a new level in a class, you get its features for that level. A few features have additional rules when you’re multiclassing. Check the information about multiclassing included in each of your classes’ descriptions.

Special rules apply to Extra Attack, Spellcasting, and features (such as Unarmored Defense) that give you alternative ways to calculate your Armor Class.

 which means that rule does not apply to this spell.

Quote from KingKronosTheTimeGod >>

Also those rules you are referring to are specifically in the Multiclassing, class features section and specifically references the Monk's Unarmored defense and the Sorcerer’s Draconic Resilience feature which both say "your base Armor Class equals" compared to this spells "the target’s base AC becomes".

No, the rule I'm referring to is the Rules Glossary's definition of "Armor Class", which you can find here. It applies to anything with an Armor Class and is not specific to multiclass characters.

Thank you for clarifying the rules you were referencing and I'm sorry for assuming they came from the multiclassing rules, I have been looking at those recently and recognized the wording. 

these rules are ment specifically about "rules", such as a player ability or natural armor and say,

An Armor Class (AC) is the target number for an attack roll. AC represents how difficult it is to hit a target.

Your base AC calculation is 10 plus your Dexterity modifier. If a rule gives you another base AC calculation, you choose which calculation to use; you can’t use more than one. See also “Attack Roll.”

 

This spell is not a rule, it is a spell that says your "base AC becomes", it is not a way to calculate your AC it is setting your AC. This rule is referring to things like natural armor, and if you want to chose the alternative 10+DEX I will happily let you except for the fact that you can't if you agreed to have this spell cast on you. This is a spell that sets your AC, not actual armor or anything else. The players handbook does make a clear distinction of rules and spells if you look at it.

My original comment is something you could use maybe once where you supprize your DM and they don't allow this at their table, or they just are more careful when roleplaying enemies.