There's no reason we should fight on this one. On a collecting quest, where there are so many boar's jaws to go around, I understand it's every Orc for himself-- in that case, I'll usually move on to greener pastures. But if we're both here for just the kill, why would you refuse a member of your own faction? This one's in Mystic Worlds' helpful Outland tips, too: if you're both camping the same named, group up.
Of course, this only goes for factions. I'll steal a kill from any Alliance I see out there, and I just expect those smarmy Night Elves to do the same to me (and I'll take them out when they're halfway down for doing it, too). But if members of the same faction are going after the same kill (or kills) for a quest reward, people, let's just all get along.