Remember that Luke Smith e3 interview? (Not the controversial one, a different one on TTK) "how do you feel about people using the same character 3 time instead of trying every class" Luke: "this is something I want to talk about so bad because I have such a passionate opinion on it, but I just can't voice it right now". Point is, it's painfully obvious that bungie developers dont want duplicate class characters, but also can't really stop players. So having exclusive guns is an attempt to get players to use all 3 classes. It makes perfect sense if you connect the dots.