Limiting what your character can do is the very point of classes - that's because, unlike Oblivion where you play by yourself, you are meant to play with a party. One of ways to encourage teamplay is to give various teammembers different elements that they need to combine to succeed. As such holy trinity was born (tank, DPS, healer) and D&D works like that. By choosing class, you choose what set of skills you want tyour character to have and you fill missing pieces with teammates.
