That may be so Victoria. The problem I have with this persists, however; you have to be online, on their server, to play solo. And even then, you also have to turn off the option for random friends on Battle.net to be able to join your single-player game if you don't want them to.
I can see the benefits of this; if you want to play co-op, this is now very easy... *IF* you get the option to allow/deny anyone that wants to join your game, instead of randomly being swarmed by other players in your single-player game.
On the other hand, your play is still Dependant on the server's stability or traffic, you can still lag in your single-player game, because the game was designed to be online all the time.
Would it really have killed them to make the option for going offline and playing the single-player entirely single, and install a hard barrier between offline toons and online toons? This way, if you want to import your single-player toon to a multi-player match, or utilize the auction-house, buying items for RL money, you have to play that toon online.
And while you play your single-player game, not only are you at the mercy of the server's stability and traffic, you are contributing to it's traffic even when playing alone. There could be several hundred thousand players online, and the ones among them who play together gets an equal share of the immense lag (if any) because thousands more play solo and add to the traffic, even if this should and could be wholly possible to do offline where the only limitation to your enjoyment is your own hardware, and you are not adding to the traffic for the one's playing multi-player.
It seems like a really poor choice to me, and adds to the huge problem of server traffic. No wonder their day-one and day-two work-days was putting out the electrical fires in burning server computers, as both the single and multi parts of the player-base combined to fry their hardware. All for the sake, of selling pixels and stat-boosters for RL money.
I used to be naive enough to think that gaming becoming mainstream was a good thing.