Just like BJJ, fighting in general is about choosing the right option for the situation at the right time. Obviously, the right option about 95% of the time is: try to diffuse the situation or just bounce. This question falls in the other 5%, though, when you can't do that. So, in those situations, again, you try to find the right decision for the situation.
For me, it's process of elimination. For example, are there multiple guys? Is anyone on my side? Unless it's 1v1 or I have very favorable numbers behind me, BJJ is pretty much out. Even then, though, if I get tackled, I'm going to defend myself, and a lot of the time, the best option for that will be BJJ, at least until I can get up. The point is, short of a 1v1 fight on carpet or grass or something, I would never try to pull RG as a goal. But in any fight, if I wound up on my back and could not get up, there's a decent chance I'd use it for a bit. I could see getting knocked down in a brawl, covering up, and one guy jumps on you. Decent chances are, if others are still fighting on the feet, nobody else goes straight for you. You use RG to defend a flurry of sloppy GnP, go for a quick sub, and then get up and either back into the feet on your feet, or hopefully, get the hell out of there.