I get the idea of making some of the lesser used jewels being more useful, but I don't think there is anything wrong with the jewel system or any of the jewels right now. The idea of limiting what jewels you could put on an item could be a mess to implement and it would piss a lot of players off. Factors like what class you are, the difficulty of the game, and the stat system's caps are what makes some jewels seem not very useful.