Sure, Microsoft could have built a skype clone in-house for far less than $8bn. But would people actually sign up and use it - to the tune of 124 million people per month? My guess is no, at least not for several quarters.
Yes. MS has a massive installed base. If they can't leverage that to get people to use it, they are clearly incompetent.
$8.5B influential? Honestly, I don't see what $8.5B gets you that investing $500M in a Skype partnership wouldn't have.
I'd like to think they have some grand plan, but honestly I don't think so.
I think they're just sitting on a stack of cash and getting pressure from investors to spend the cash and grow.