There's a little bit of numerical chicanery going on here. Allow me to present some equally valid counterexamples:
Clr3/Wiz3/MT5 vs Clr 11
HP: 3d8+8d4 VS. 11d8 - Theurge behind by about 16hp
BAB: +5 VS. +8/+3 - behind one attack and 3 BAB
Saves: +5/+3/+10 VS. +7/+3/+7 - 2 Fort behind, 3 Will up
Well, the Cleric looks to be a bit ahead here. He'll get more mileage out of his armour proficiency, and the Will Save gap won't be that noticeable given the requisite high Wisdom.
The Theurge does have a familiar, though not a particularly potent one, and gets Scribe Scroll as a freebie; whereas the Cleric has 8 extra levels of turning power and almost certainly gets more use out of their Domain powers.
Spells:
The Mystic Theurge can cast (assuming he's a specialist Wizard):
Cleric (8th): 6 / 4+D / 3+D / 3+D / 2+D
Wizard (8th): 4 / 4+S / 3+S / 3+S / 2+S
A grand total of 42 spells, with 79 spell levels between them (Orisons counted as 0.5 spell levels).
The Cleric can cast:
Cleric (11th): 6 / 5+D / 4+D / 4+D / 3+D / 2+D / 1+D
A grand total of 31 spells, with 77 spell levels between them (Orisons counted as 0.5 spell levels again).
Now, Technik4 would want us to discard Orisons altogether and pay close attention to spells above 3rd level, which would put the comprison at:
MT 32 spells, 74 spell levels, 6 spells above 3rd level.
Cl 25 spells, 74 spell levels, 9 spells above 3rd level.
It would appear that the Cleric can cast 10 to 25 more different spells than the Mystic Theurge, depending on scroll access...
Could it be that the humble Cleric is more versatile than the Mystic Theurge, along with all of his other advantages (hp, no armour dilemma, BAB, high level spells)? Say it ain't so!
Well, let's try looking at something less disturbing... we all know that the Mystic Theurge is at least as good as a Wizard, right? Right?
Clr3/Wiz3/MT9 vs Wiz15
HP: 3d8+12d4 VS. 15d4 (MT ahead by 8 or so)
BAB: +7/+2 VS. +7/+2
Saves: +7/+5/+12 VS. +5/+5/+9 (ahh. clear superiority.)
Yeah, the MT's got a little hp margin and much nicer saves than the wizard. This is going to be a walkover
Now, to be fair, the Wizard's familiar has an extra +6 Natural armour, 6 more Intelligence, Spell Resistance, etc, etc, but "My familiar can beat up your familiar!" isn't worth much in a Mage Duel
The Wizard does have 3 bonus feats, mind you, and that's quite important.
Spells:
The Mystic Theurge can cast (again, assuming specialisation):
Cleric (12th) 6 / 5+D / 4+D / 4+D / 3+D / 3+D / 2+D
Wizard (12th) 4 / 4+S / 4+S / 4+S / 3+S / 3+S / 2+S
For a phenomenal total of 63 spells, with 174 spell levels between them. If we pare off the Orisons, that's down to 53, with a "mere" 169 spell levels. I figure, at 15th Character Level, that the important stuff is going to be 5th level and above, and here the Mystic Theurge only has 14 spells to contribute at these levels.
The Wizard can cast (as a specialist):
Wizard (15th): 4 / 4+S / 4+S / 4+S / 4+S / 4+S / 3+S / 2+S / 1+S
That's only 38 spells, and 138 spell levels. Again, cutting away Orisons, down to 136 spell levels, but 14 spells at 5th level and above.
Well. It's hardly conclusive. While the MT may be able to throw around a lot of different spells, they don't have any numerical superiority in the heavy-hitters, and those they do have are lower level. I'm also inclined to believe that I was being somewhat generous in counting 5th level spells, but that's debatable.
So, really, the Mystic Theurge is only showing itself to be a decent alternative to a mono-Wizard. If I'd gone a-raiding Prestige Classes (Loremaster, Archmage... Red Wizard
), the MT would be looking decidedly less competitive.
On the other hand, I specifically chose levels where the MT would be disfavoured on break-points... but reapersaurus chose a level where it'd be favoured on break points. All's fair, I guess.