I think the big selling - or buying point is the subscribers.
Combining MSN and Skype gives MS a massive user base.
Apart from that I don't see the point of buying Skype when they could have easily improved what they already have, MSN. What can Skype do that MS couldn't get going on MSN?
But as Squillian mentioned Facebook is looking to dominate this video calling business and they have huge userbase.