I'm pretty sure the origin of the term Ninja (as inL: ninjaloot) comes from MMO's where loot wasn't locked to the group that killed it, especially when the mobs weren't in instances. So your group could kill Mob x, and some random guy could swoop in and loot it quickly and vanish (like a ninja).
There are a fair amount of people who seem to be afraid to admit to this (that is: the fact that they allow their concept to be influenced by the mechanics).