Exotics are too unique and powerful, and certain combinations would be pretty game-breaking (and already can be, even with only one exotic being allowed per "side" of your character). Bungie would have to make the enemies in the game way too strong (back to the bullet sponges of old) AND would also have to work much [i]much[/i] harder on game balance (to fix unforeseen combinations). Also, as others have mentioned, this would make legendary items useless, and would make getting them pretty unrewarding. As it is, things like raid armor and "unique" purple weapons are really cool, because you know you can use them with no restrictions. For example, my Horror Story is a godsend, because it's a very good weapon that doesn't hamstring my use of an exotic secondary or heavy.