Azhdar: GOG just needs more titles to attract new members. Some Steam users are interested to have DRM-free version of their Steam libraries. So if they see those games on GOG, they will come here.
I would think that the Steam users who want DRM-free copies of their games would have already bought them here on GOG (at least, for the games that are available on both services).
As for any exodus of Steam users...it would be nice, on one hand, I suppose. On the other hand, there are vocal hardliners who despise the idea of DRM-free games. And yes, I know I sound like an elitist when I say this, but I'm not sure if GOG getting another flood of Steam users is necessarily a good thing. Anyone remember what happened when the Alan Wake games came to GOG?