A couple of things come to mind when I put on my spinmetal bs detector hat... Bungie must have seen that more than a few players were using loads of weapons parts to level up factions quicker than anticipated. So, nerf. Or, bungie saw that many players were having fun upgrading all kinds of stuff and playing the game the way they want to. Oops, nerf. Or, bungie has a spin wheel on the wall and every week some intern gets to play "spin for nerf" and this week it was weapon parts!