You can read a lot of theories why Microsoft spend 8.5 Billion $ on Skype. Either it was a pure defensive play to keep it out of the hands of Google and to some extent Facebook (even though Microsoft is a stakeholder in Facebook) or it was to buy up something close to a social network (however very much focused on the topic of online communication).
It seems that there is no real need from a technology point of view as Microsoft has the technology for Audio and Video Calls today in different iterations (like the Microsoft Live Messenger Network, Micrsoft Lync in the Enterprise space and Lync as part of Office 365 as a Cloud Service). Also the number of users on Microsoft Live Messenger is almost double of the active number of Skype Users.
I believe there are two reasons which led to the decision to takeover Skype: First Skype is a verb and second Skype will extend the reach of Microsoft's Enterprise Offerings to almost anyone on the Internet and beyond (thanks to Skype’s dial out features which are not available for Live Messenger).
Any brand that becomes a Verb is a good brand and worth having. Not very often someone says to me let’s Live Messenger or lets Lync – people say let’s Skype. If Microsoft is able to extend this Brand and re-brand their Live Messenger network as well as their Enterprise Offerings (on-premise and in the cloud) we are suddenly talking about close to 2 Billion people Skyping.
Regarding the Reach of Microsoft’s Enterprise Offerings Skype offers the last missing link. Today public federation of on-premise and cloud services based on Office Communication Server or Lync Server offers connection to AOL, Yahoo, Live Messenger, ICQ and Google Talk – Skype is the only remaining mayor service missing. With this added many business calls and related cost will vanish – Telephone cost are still a mayor issue for corporations and this will position Microsoft Lync even more than today as the predominant VoIP PBX for Enterprises.
Altogether I think this was a very smart move and will enhance Microsoft’s products and reputation especially in the fast growing Unified Communications market space.