Well as for AD&D, dual class and multiclass characters had it far worse than they do in 3.x. It was even harder to be effective.
Your prestige class would be far more powerful than others, which was admittedly your plan. Picking any prestige other than yours would be suboptimal, if not foolish.
The six levels you need to get into yours is about the same for many. The attempt at averaging does little to balance anything, as most prestige classes have done this type of thing already. Try stacking a fighter with anything and you get a virtual feat/ability monster for only a three level hit. Try stacking two spellcasting classes. With a few good feats like practiced spellcaster, you are so far beyond the Mystic Theurge that it could overwhelm a game. Compare a wimpy 20 level regular build with your prestiged monster. No contest. Staying true to one class wouldn't be the power gamers choice (read: everyone who isn't a sniveling role elitist looking purely for flavor).