MS has had its own video conferencing via MSN messenger for years. I don't see the value of spending 8.5 billion in order to get Skype's tech - I would think that money would be better spent on a very fat pipe on which to run the live video.
2. Skype has 110m+ active users, and 660m+ overall users.
$8.5b is too much, in my opinion, but Microsoft obviously believe that it's a fair price to pay for the reasons above (among others, of course). The value gained is tremendous. What Microsoft does with that value remains to be seen.