M1A, there's good reason why people do that. They are personalizing their AR15, which is one of the major benefits of the rifle. For many and exact color match is not an issue. If you're a collector it may be. if you're a shooter you probably don't pay too much attention to it.
Many get an upper they like - the way it shoots, it's reliability and would just like to keep it.
Azalin, you're off the mark - you [i]need[/i] 2 uppers for each lower. [;)] I like to have one CQB upper and one accurate shooter upper for each lower. Hey, then you're well covered, right?
And there are also the cases where folks need a few hundred bucks fast cash but just don't want to part with the whole weapon. Again, just another benefit of the AR15 rifle class.
Shoot well.
CMOS