To be fair, it is sort of needed, as without that limitation, you could do things like take southern magician at level 3, use your skill points appropriately, then take Mystic Theurge to gain 2 caster levels per level.(So, say, by level 13 you would be casting as a level 23 caster.). Which is a bit on the OP side.

(Yes, spells known/per level max out at level 20, so it wouldn't be as game breaking in this game as it might be in lower level games, but with a little bit of effort you could get a fairly excessive CL and break things with holy word like spells)