One of the succeses of the refined D&D class system is that many distinct mechanics and spells are good, so players can choose what is fun and not google "best build" (although I guess you can rob yourself of fun if you really want to). A skill system that is too open can remove variability and diversity which makes campaigns feel repetitive and boring. So the class design needs to have some way of keeping characters distinct and allowing many different pathways to be effective.
