@Muggie, if you are talking about taking 2 spellcasting classes and having a cap at something like level 12 [using a theruge prc] then it basically works out like this:
Cleric 3/Wizard 3/Mystic theruge 6 (Assuming 20 in both casting stats)
Casts 5th level Wizard and Cleric Spells
# of spell levels: 112
VS
Wizard 12 (assuming 20 in primary casting stat)
Casts 6th level Wizard spells
# of spell levels: 102
The Wizard will be more powerful [able to cast 6th level spells] and he will have more uses of his higher level spells, but he will have less diversity than the theruge and fewer low level spells. Not only, the Wizard also gets access to new levels of spellcasting a whole level earlier.
So yes, Id say it is viable at low levels, but then I also think you'd miss that next level you'd usually have. And you'd never be as good as a pure wizard or a pure cleric. But if your party has neither of those, then sure. Why not.
However, if you are talking about just taking a few levels of a several casting classes in a game that caps off at 6th or something. Then no, not without some way to massively boost your caster level and handle the MAD. (And yes, Im sure somebody somewhere could figure out a way).