What are you doing to make sure that the cleric and other magic users don't step on the toes of the other classes? If a cleric can be sneaky and use a bow, what place does the ranger or rogue have?
It's important to remember when talking about competence in particular areas that there is a distinction between being good at something and being the best at something. We want to make sure that each character class shines in certain arenas, and as a result while you might build a cleric who can sneak and use a bow (to use your example), and your cleric might be very good at those things, the ranger or rogue will probably still be better. We want to give plenty of flexibility for people to be able to build the characters they want to build, that are good at the things they want them to be good at, while still providing ways for all the classes to have certain realms in which they are the best.