Antoine said:
What about a wizard cohort for that Mystic Theurge?
Ah, but the wizard spent his feat on leadership while the MT spent his on practiced spellcaster. The MT could spend another feat to get leadership, but the MT is fast running out of feats, a scarce resource. Unless, of course, you go wizard all the way and get 2 more of them.
I am not saying that the MT is unplayable, so long as he takes his practiced spellcaster feat. He only sucks a little bit then (although he sucks worse at low levels before getting to MT). But by no stretch of the imagination should anyone think that "practiced spellcaster" actually makes a multiclass spellcaster outshine a single-class spellcaster. Ain't gonna happen. At best, it allows people to take a hit to their character's effectiveness for roleplay purposes (nothing wrong with that in and of itself, so long as the hit is small), as opposed to being the Gleep the Supermonkey (for you Justice League fans) of the party.
That is, with the feat, you still suck. But you suck less. In fact, your suckage is small enough that you could play the character.
[rant]But without Practised Spellcaster, you suck so bad, that you actually are in danger of bringing the party down with you if the DM forgets to take your suckiness into account when looking at the CR of monsters to throw at the party.
As a multi-class spellcaster without that feat you become a black hole of suckage. The supersucker. The nadir of playability. The rotting corpse of the party mascot, dragged along by a knotted rope tied to your heel. The joke of the Gawds. Fodder for Dire Housecats. A suckiness bespeaking of a self-destructive tendency in the character, and possibly, the player. Wile E. Coyote, with none of the charm. If WOTC's optimization boards had a contest for what would be the LEAST effective non-NPC class character, you would be at the top of the list (Let's see, PHB only: half-elf cleric 5/wizard 5/druid 5/sorceror 5. There we go). Jack Chick would start giving you advice on how to optimize your character. Mothers would warn their children to be good or they might end up like you. Yeah, THAT bad.
You NEED that feat if you play a multiclass spellcaster, or your DM has to go into Care Bear mode with the monsters. In which case the other characters will have a cake walk, because they DON'T suck.* And then the game will suffer. So at this point, your mechanically bad character, because he lacks that feat, actually interferes with the roleplay/mechanical enjoyment of other characters. And it will be ALL YOUR FAULT!!!!!

[/rant]
My above rant does not begin to do it justice. But it was therapeutic, so it's all good.
*Exception: If ALL the characters are multi-class spellcasters without the practiced spellcaster feat, AND the DM goes into Care Bear mode, it just might be playable. Unusual, but playable.