Getting a second spellcasting only a couple of levels behind your primary spellcasting class without any set-backs is pretty damn good. An Azata mystic theurge using Favorable Magic, Zippy Magic, and anything which improves spell penetration could have some serious endurance. No more would you be filling time between when you need to cast spells. You could simply cast every action from dawn till dusk. Going mystic theurge would effectively allow you to double your spellbook without any loss of CL. If abundant casting applies to all casting classes, and not one, then it's even crazier. A character could end up with something like quadruple spells, that cast twice when single target, at which enemies have disadvantage. All at no loss to CL.
It's not 'only a couple of levels behind' it's delaying your spells for your primary class by at least three (or possibly 4) levels. Unless you're an Angel or a Lich with merged books you will never get access to level 9 spells. It will have the azata 'pseudo rest' spell to keep you casting forever, I suppose, but losing out -3 or -4 on caster level checks will hurt against a lot of magic resistant targets, even if you have spell penetration.
If you really want to go for it, then an Angel oracle (4-6)/sorcerer (4-6)/mystic theurge 10 with merged spell books would probably be more effective than an azata, as would a Lich Sorcerer (4-6)/Oracle (4-6)/MT 10.