Thanks and Apologies
I'm sorry I lied to you all about the topic of this thread. I wholeheartedly apologize, but I felt that it was needed for the point I mean to make now.
Your powergamer's advice has been very valuable, and thanks to Nifft, I now have an answer to an unrelated question I've had for some time.
The fact of the matter is that this thread was founded on a lie. I have not been invited to join a new game, but now I kind of wish that I had been. I'm now itching to play a druid.
The fact of the matter is this:
This thread is and always was about the Mystic Theurge.
Some say it, and other classes like it, are overpowered. Some say underpowered. I fall into the latter camp.
I asked above if a 9th level druid was more powerful than one 7th level wizard and one 7th level cleric. Nobody came forward to say that it wasn't.
Can we all agree that one 7th level wizard and one 7th level cleric is more powerful than one 9th level Wizard/Cleric/Mystic Theurge?
I propose that prestige classes which offer 1/1 spellcasting advancement in two classes can also offer some special abilities without being overpowered. It is clear to me that needing levels in more than one spellcasting class is the most severe, steepest requirement that any prestige class can have, because no matter how cool the benefits of the class are, this will cripple any character, if only from a powergaming perspective, which, to my mind is the only perspective that matters where game balance is concerned.