Network effect
All the negative comments would be applicable if Microsoft had just bought Java, but Skype is more like Hotmail.
First, Skype strength is in the network effect of ubiquitous access.. shutting down Skype on {Linux; MacOS; Android; iOS; Symbian} would be counter-productive because it would reduce the appeal in general.. even SteveB knows this and said as much at the announcement.
Second, Skype Video conferencing performance is more a function network performance.. most current broadband is asymmetric (you might get 50Mbs download, but only 512kbs up).. it’s the upload speed that determines quality.
Third, the Skype ecosystem extends into POTS/PABX telephone system, and offers the opportunity to route desk-phones to mobile devices and vice-versa, offering Telco’s the chance to charge for mobile services that are actually routed via Broadband.. especially in saturated urban area.