I'll admit that, while powerful, the Mystic Theurge is not totally unbalanced. However, what would you say about a Mystic Theurge with very powerful class abilities at every level in addition to the double caster progression? The True Necromancer does get 8th and 9th level spells as class abilities, which he can cast for free 1-2 times per day each. What other PrC gets class abilities as powerful as Wail of the Banshee (all in area save or die) and Horrid Wilting (19d6 damage to all in area at 20th lvl)? And the Necromantic Prowess ability means that by 20th level, he casts necro spells as a 19th level Arcane/Divine caster! Unlike the Theurge, he also still advances in his Rebuke Undead power, and he acts as if he always has the Desecrate spell active. In a single spellcaster, these powers may be balanced, but the tradition of double caster advancement (Mystic Theurge, Psychic Theurge, and Cerebromancer) has always had absolutely no class abilities at all... no bonus powers or feats of any kind, since the double caster progression is plenty. Not only does the True Nec break that tradition, it does so in a big way...