A roleplaying game is about obtaining huge amounts of power. You play to amass experience points and level up, and you want to feel rewarded for everything you do. For some reason, Dungeon Siege III doesn't agree, and has therefore implemented what may be the most useless, unappealing co-op mode in all of gaming.
Dungeon Siege III's co-op will not provide experience points to anybody joining another person's game, as they will be viewed simply as "assistants" to the host, with nothing to gain. Apparently, Obsidian believes that gamers are thoroughly altruistic individuals, who will waste hours of their time just to help some random ballbag online.
A single player controls a party of four characters, and up to three human brains can drop in at any time and take over one of the members. Good luck finding anyone though, because I know I wouldn't want to play an RPG for absolutely no personal gain.
Any philanthropists out there who are less selfish than I am?