If they could separate out the business logic (communicating with the network, using proprietary protocals, etc) from the interface logic (UI, access to local audio-in/out, access to local video-in/out) it would be a major boon. Though open-sourcing the whole thing might be nice, it could have unexpected consequences (easier to filter Skype with DPI, allow NSA/CIA/etc to tap calls, etc).
The value in this would be that we essentially get a published interface to that library.
It would be a really good move though. For Skype on all platforms.