This is a good idea. I'd honestly like to be able to level my commons up as well though, from common to uncommon, to rare, and then to Legendary. Even if we couldn't make them Exotic it would be an amazing feature to level up old weapons and gear because you preferred how they performed.
English
-
While I think it's a good idea to be able to level any weapon I think its too much effort to impliment it for all weapons. Maybe being able to start from rare and go up but in most case anything in white or green can be replaced with something of superior performance in the rare category. Unfortunately I often see things in the rare category that are never represented in legendary. Again if they could do it for every weapon great, but so many are easy to replace and quick to upgrade it may not be worthwhile.
-
Well they'd just unlock more things on the upgrade trees. I personally think the white weapons looked the coolest because they aren't so colorful.