One of the main reasons is probably so new people coming into the game don't feel that they start at a huge disadvantage to others. Just imagine someone starting Destiny right now going into crucible and trying to get into a raid. It's not just the lack of knowledge but the gear gap with weapons will set them back awhile. I see both sides but can't settle on one. I'm a day one player and would not like to see my weapons and armour become obsolete, unfortunately it is a staple for mmo's