It puts you in large online lobbies where you mostly ignore everyone. If you're playing alone it could be ok but it might kick you out. Can't say i experienced that when i played the beta.
I agree though, it definitely isn't designed for SP at all. 1 of the bosses in particular i remember doing it on SP and spending the whole thing getting my ass handed to me (1 boss with a 'suction' attack that pulls you towards it, not a problem, but when injured it splits into 2, then again later into 3, all of which can suction you over and over).
Doesn't help because the balance is so screwed up with multiple people. Arakna, a hero i could walk through solo with, wound up getting 2 shotted by some random frigging enemy...