Those rerunning dungeons multiple times to get a certain loot is probably glamour hunting. Personally if you out level a dungeon I think it's best to move on unless you're disgustingly undergeared. GC sells gear sets all the way up
the level bracket until level 50 (for now) so you can use that to replace an under geared piece. You could also ask a friend to craft you HQ gear since low level stuff isn't hard to make. Unless you're after for glamour, I wouldn't bother out leveling a dungeon just for a piece of gear that will get replaced in the next dungeon.