I would strongly disagree, some classes may be, well, colorless and some cases weaker whitout anysort of PrC available. But I can understand if you want limit them stirctly to core + racial paragon classes, but I could provide full info of the Knight Protector PrC if that is the case. Examples of plain classes which would be weaker whitout PrC are examply sorcerrors, a plain pure sorcerror is weaker than wizard of similar level in many ways (if we use strictly core one). Paladins do nto get anything special after 10lvl and many would multiclass by then or earlier.
DMG provides many PrC which do not need stirct training to become a one, examply Mystic Theurge I have hard time to imagine to specifically 'train' to be Mystic Theurge. The virtue of multiclassing alone is enough to qualify for the class. Archmage is good PrC for sorcerrors and Dragon Dicipline also (I asume there are dragons in the Maze what wanna be Dragon Dicipline can study).
Yoou could rule that we must provide any non-core PrC either a link or PM full details of the PrC for approval and no one cannot take more than one PrC examply.