Windows Live Messenger is only popular for text IM,but not for video and audio chats. The quality (and framerate) of video calls with messenger is nowhere near as good as Skypes, and the UI of Messenger is designed for text rather than visual communication: you only get a small video window off to the side, rather than a huge "in your face" Skype view.
Windows live messenger 2011 has a real 'in your face' view for video conferencing and does support HD webcams. My personal experience has been better with the Messenger framerates then the Skype ones. Putting calls on hold and conference calls with more than 2 people are sitll missing though. I personally also like the UI of Messenger Much more than the new ugly skype.
Also, what's in this for the Android and Linux clients? It's enough to expect Microsoft to give lip-service support to the Apple crowd, but Microsoft has never officially acknowledged Linux' existence (beyond the time they were legally obliged to release their Hyper-V drivers under the GPL).
They have promissed to keep the skype brand name, and keep supporting other non-microsoft platforms.