It seems like they're just flipping a coin to decide what games to put on sale. You'd think they would be smarter and push their exclusives more, like the viva pinata and age of empires sales.
I think they are trying to get people to use the client that normally wouldn't. Those that are interested in MS titles don't have much of a choice. They use it or don't play it. I still think they would benefit from putting some of them on sale though. When's the last time they even published a major PC title? AoEIII? All of their studios have left, were sold, or shutdown. I don't see them having a ton of exclusives anymore and I have a hard time believing they are serious about PC gaming even with sales like this.
If they put 1/10th of the R & D they put into xbox into GfW it would be a much more seemless experience. I would be happy with a simple friends/chat/im client that I could run on my pc in the background and see what people are doing/playing but they haven't even provided that.