Refreshing though.
I can think of a handful of players who do it at every opportunity. I think it's just a sort of "I play to win" mentality, so they feel stupid if they don't take everything they might possibly be able to use, or take that opportunity to cripple their enemy a while.
I'm not entirely against it, makes sense, but I don't do it myself. I usually even leave weapons I'm vulnerable to, unless you're just really raping me with them.
Make me really hate you, and I might resolve to full loot you. In those cases I'm normally over it by the time you die though. But I can't imagine this game without the option of full looting.