Online only has always been about trying to combat piracy in my opinion. I think any other lip service it gets is kind of silly.
Piracy is a *huge* issue with anything digital, and I don't blame them one bit for trying to do anything they can to try and protect their property.
Piracy is for sure a huge reason.
But not the only one. In the past some developers even talked about that (not from Blizzard though, but it doesnt really make I difference I guess).
They hate people selling their "used" games for example. Its someone buying a copy of the game and you dont get any money for that. A lot of people in the industry, shockingly, believe thats close to "stealing" their game ... wow! They dont get a penny! What a shame! Imagine if the rest of the world would work the same. Sorry! Cant sell your car! With always online DRMs and accounts etc. you can almost eliminate that.
Another reason is, to put it simply, controll. With accounts and players registered on your servers, you know how many play your game, what they play, how often the play, when they go to the bathroom, how often they have sex etc. you get the idea. They can get a lot of data and informations. But thats not even the problem. The issue starts when a company, like Blizzard, or EA believes that youre not "good" for their game. So that the "rights" they give you will sudenly be ... gone. Imagine for what ever reason (I will not go in to details here) your account is banned. As said, lets not discuss why it happend, could be as well some kind of accident, or what ever. But its a fact that now you have a 50 dollar brick on your desk and there is nothing you can do with it and just "hope" that Blizzard will eventually allow you again to play their game again if you say often enough "please". It sure is rare that accidents happen, but it does happen. No software or human is perfect.
The way how I see it, as consumer not as developer! Always online DRM measures have a lot more negative aspects to it then positive sides, particularls since, from the consumer perspective, there is nothing that says I cant get the advantages together with offline, for example, steam. It offers offline modes. Or games could make seperate worlds, where you buy the game, play the single player and have never to bother your self with accounts, patches etc. But then they would lose a lot of people, people like me, that see no reason in creating accounts for 6 different companies ... which I will never ever use again.
And I didnt even mention "issues" here. Like accounts that wont let you login, I avoid games for windows life for that reason, I always had issues with that software ... nothing is worse to be stuck at the loggin screen because THEIR servers have issues and you cant play a game you payed for now, and its not even an issue with the GAME, but the damn loggin software ... that wants you to be online ...