a) name recognition - Skype is a premier name in conferencing
b) eliminate competition - gets rid of the biggest tool out there used by people
c) gets them info - Skype's info is now MSes
d) affect competition in other markets - MS can deter use in Linux, eliminate it, or use it as a foothold to affect certain aspects of it.

Remember, Gates got into bed with Adobe and HP early to affect how the printing world worked in the early days of GUI-based PCs.

This might be Ballmer's way to get a recognized brand to use, as well as be able to affect how voice/video conferencing on Linux for the masses is done. You never know what MS will try to pull when they get their foot in the door somewhere.