In my view, this deal is all about buying brand awareness. Despite the fact that Microsoft have similar technology in their Live Messenger products for consumers, Live Messenger is not top of mind for consumers when they want to place a VOIP voice or video call. If Microsoft spent the time and money to integrate Live Messenger video/voice into their product lines such as Xbox and Windows Phone, they wouldn’t see the return on that because most people would just say “but all my friends are on Skype!” and would not use it.
By purchasing Skype, Microsoft can integrate it into all their products (including their business-focused Lync) and have a built-in user base and user mind-share. It will sell more products because people who already use Skype will consider purchasing an Xbox, Windows Phone, etc. because they’ll realize that they can use Skype to connect to all their friends and family by using it.
Additionally, when the LTE networks really start to come online and become the primary network being used by the operators, Microsoft will have a big hammer to swing as the owner of the most prevalent VOIP technology with the largest existing userbase. They can license that technology to the carriers for use as their VOIP technology that will be riding on top of LTE (and future networks).
All in all a very smart move in my opinion. It’s likely not a huge direct revenue generator, but a revenue enabler across their whole range of products and services.