They're only central R&D because MS employ most of the paid dev resources working on it and no one would use any forks except from MS (or possibly Google, but they wouldn't ratify MS's platforms by investing resources in maintaining a fork) so there's no risk of fragmentation.

That definitely doesn't apply to all projects, you can look at the successful forks of Oracle's OSS acquired products.

If the OSS'ed code-base is valuable, you've just given a open invitation to all your competitors to create their own competing forks.