No, because it's a total mess and leads to some... interesting cases, to say the least. You know, where you have a creature with 4 HD and 4 levels of Wizard, and you could either take a level in Wizard (it's associated due to being more than the HD), or take another 2 HD to increase the CR by 1, then next CR+ take another 2 levels...<br/><br/>Besides, if that should work for monsters, it should work for players, too. Currently it's just "Hah, you took a class that doesn't synergise with the rest of your abilities? HA! Suck it!"<br/><br/>(Not that I haven't used non-associated class levels for making some NPCs challenging)