Microsoft already has the technology necessary in their own audio/video/text Windows Live Messenger platform. So I don't think it's about that. And yes, I feel sorry for the Skype staff today -- I don't think this move bodes well for them at all. Their competence may not be what Microsoft is looking for here.
And as for other reasons, the paying customer base (compared to the non-paying WLM user base) of Skype could perhaps be attractive to Microsoft. Keep in mind that Skype is running with losses despite all these users, though.
In the end, taking all these thoughts together, I can only imagine that this is a risky move by Microsoft. I think they are hoping for awesome synergy effects from some forthcoming integration with their products. I assume something big, and no minor idea, since it needs to pay these $8.5 billion and more.
My first idea was integrating this with Windows Phone 7 (8? 9?) to get phone calls at data rates, but I have no idea how they'll going to get the providers to accept that. That would be a feat as grand as Steve Jobs getting the music companies to sign on to iTunes back in the days, if not greater.
Otherwise... Hmm, someone mentioned Xbox or Kinect integration to communicate with others with these devices... Well that's a thought but why shouldn't they be able to just implement that feature with their Live network? Write a WLM client for these - done. No $8.5 billion wasted.
Not sure if there are other ideas about where MS may be going with this?