Thank you all for the explanations. But, basically rbanffy is right ?
According to that email and what I read here, they want to split just because GNOME promoted proprietary software on their blog ?
But there's one thing I still don't get. Even if they split, what's the big deal ?
It would send a message - that the developers of Gnome don't think being Stallman-like free is not a big deal.
If, however, they do entertain such toughts, I will start using the "free as in Stallman" fork that will, undoubtedly, become available the week following the inclusion of any not-very-free software and that will be maintained by many of the same developers who do it today, with a lot of common code between forks.
But there's one thing I still don't get. Even if they split, what's the big deal ?