Man, that sounds so easy and I'm sure it never has issues. Also, having to install and set up all their little app stores, figuring out which apps have or lack which features that you like, managing your library between WB's app and Activision's app and Capcom's app (soon to be canceled, along with all your digital purchases on it) and SquareEnix's app...
Are you citing guaranteed "features" in clients I suggested hypothetically? If you're talking about Ubisoft, they were literally all about Always-Online DRM for quite a while. I'm not going to hold my breath on that always being guaranteed. Besides, that doesn't mean other features of the service won't be down. It's already a pain keeping track of or finding out that Steam has scheduled downtime, or PSN does, or XBLA, or Nintendo Network, or Origin, etc. God knows how much downtime some third party game services would go through the first few times they hit serious troubles.
Ubisoft's ahead of the curve in that regard, and their app is STILL crap. EA's the only one that may be able to justify their service, and only because they started implementing some very forward-thinking policies and gave away a lot of free crap. They're competing with Steam with desired features in that regard, not just holding their games hostage to their own little proprietary service and making the consumer jump through extra hoops to appease them. Although they tried that for a while too.