I don't train in BJJ (not yet anyways) but in my opinion the rubber guard is such a great system and it is highly under rated. It does require some flexibility, and there is nothing preventing you from being flexible. Like most things it takes work to get there, and if your dedicated you can be just as flexible.
In Eddie's book MTRG, he gives an example of a guy going into a kickboxing school and coming to the realization that he can't kick anyone in head but he doesn't write the sport off. He doesn't think that the guys are genetic freaks, he just says to himself "Man if I want to kick someone in the head, I've got to stretch." In BJJ, when some people see the Rubber Guard in action they think, you have to be born flexible to pull of those kind of moves.
For those that haven't, I highly suggest picking up Eddies book Mastering the Rubber Guard. It really is a great read. Anyways here is a cool video of Eddie Bravo in action, and showing some Advanced rubber guard techniques that one of his students came up with called the Double Bagger.