Samhaine said:
I believe otherwise. I see the potential for a wizard's worth of spell slots that can be used entirely for buffs and utility, while a normal wizard would balance buffs and utility spells with options to contribute directly to combat. IMC, I believe enough this would cause a problem that I don't want to playtest it for another year and then force the player to respend his character if I determine that, indeed, I was right.
Realistically, you don't get that many useful net spells out of the combination. For example, take a typical 9th level Mystic Theurge (Cleric 3/Wizard 3/Mystic Theurge 3) vs. a 9th level single classed Cleric. Let's give them both a 20 Wisdom (accounting for a reasonable initial score, plus magic items and level adjustments), and just to make it interesting, we'll give the Mystic Theurge a 20 Intelligence.
Their spells, by level:
Cleric
Divine - 0th: 6, 1st: 6+1, 2nd: 5+1, 3rd: 4+1, 4th: 3+1, 5th: 2+1
Mystic Theurge:
Arcane - 0th: 4, 1st: 5, 2nd: 4, 3rd: 3
Divine - 0th: 5, 1st: 5+1, 2nd: 4+1, 3rd: 3+1
So, let's net this out
Cleric - 0th: 6, 1st: 6+1, 2nd: 5+1, 3rd: 4+1, 4th: 3+1, 5th: 2+1
Theurge - 0th: 9, 1st: 10+1, 2nd: 8+1, 3rd: 6+1
Net Theurge: 0th: +3, 1st: +4, 2nd: +3, 3rd: +2, 4th: -4, 5th: -3
Just going by raw numbers, the Mystic Theurge only "gains" a net of 5 spell slots per day, and if you discount the extra 0th level slots (which are trivial in general), that number drops to a net gain of +2 spell slots per day.
And those spell slots are low level to begin with.
Figure it this way: offset the MT's +2 3rd level slots, +3 2nd level slots, and 2 of the +4 1st level slots with 4th and 5th level cleric slots, and you are left with the MT gaining a net +2 1st level slots and +3 0th level slots. How many cool 1st level wizard buffs are you going to have to deal with?
And for those +3 0th and +2 1st level spells slots, the MT gives up d8 hit dice, Medium BAB, good Fort saves, turning progression, and advancement in domain abilities. He has a lower caster level with all of his spells to boot, meaning the effectiveness of his spells is hampered as well.
So far? Standing in the back, dropping AoE divine debuffs, healing the party, sometimes getting a hit off, and wearing too much armor to be the prime target of the NPCs that are actually trying to even the odds. Nothing that has happened so far would lead me to believe that being somewhat closer to a Wizard would hinder his normal method of operation. And I forsee being able to round himself out with an array of buffs to be likely to mitigate a lot of the difference between wizard and cleric.
How is he going to do that with a net +3 0th level and +2 1st level spells?