I just think it'd be a nice idea to be able to play a round with your friends on one team, rather than curb stomping them because they're totally new to the game.  Sometimes it's nice to just unwind with a few people and wail on the computers.  It's also nice to have a game style you can test strategies in without affecting your ranking.  I abuse the DotA AI maps when I'm testing build options, and usually it's invaluable for actual game play to hone your skills and perfect your new build ideas.

By extension, I suppose it wouldn't be too hard to ask if the single player campaign could be made co-op too.