Far smaller companies, with much smaller populations for their games, have their own clients. Tera, Wildstar come to mind. The notion that Bungie, backed by Activision/Blizzard money can't afford the money or the time to make their own is laughable.
Blizzard isn't owned by Activision. Blizzard and Activision are the same company.
They can of course do as they please. I just see this as a breach of consistency and breaking their own words, by saying that the app is a platform for Blizzard games. So if they want to change that, they should go all out with it, and not be dishonest about it.
Activision has the money and the capacity to make their own client if they wanted to, but they don't, because a place like Blizzards app has as you said all the work already done. And they don't want to be involved with Steam, because that'd add an extra hand into the process that they can't control or manage.
My gripe with this is that it's clearly against what Blizzard has always said, that Blizzard is Blizzard and that's all it's gonna be. I don't buy this corporate talk of "helping out". Bungie completely capable of doing it on their own.
Bungie is gonna handle everything except for log-ins, authentication and social functions. That's what makes this such meaningless "help".
I know my complaint is probably ridiculous if you don't care for principles and coherency. But I'm a real fanatic about that. Bungie doesn't need this help. They really don't. THe reasoning is complete bullshit. They want to publish this game as fast as possible and cut corners doing so, so sayeth the investors.
I would have zero issue with them putting more Activision/Blizzard games on the app, at all. I welcome it. Competition to Steam is needed. But this story of generosity is bullshit and completely dishonest.