I wouldn't say that. I'd say that single player is there for people who want the full Diablo experience. You simply don't get that on multiplayer. The lore of Diablo has always consistently stumbled over itself. Diablo1 was a great example of that. They even had different cinematics depending on which character you defeated Diablo as, each one of them shoving the soulstone into their forehead. The problem with that was that, in Diablo2, it was the Warrior who killed Diablo himself, and was consequently corrupted by Diablo. Despite having the story of Diablo1 progress in such a way that any character could have killed Diablo, in Diablo2, it's definite. Not only that but the two other characters were still incorporated into the storyline of Diablo2, despite being a complete contradiction with Diablo1. Even the gender choice for the Barbarian in Diablo3 clashes with the storyline as both share the same back story, though we blatantly know it was the male Barbarian who was in Diablo2. Regardless, the Barbarian has to have been present, as he's already done so in Diablo3.