You know, I've always wondered about this too. I do side rail for 9b, and you see pros do it too, so that's not my concern. I mean 8b back rail breaks.
I got used to breaking that way before I knew it was "wrong" and to me it feels a thousand times more comfortable. Even if your hands are dry, they will never be as perfectly smooth and frictionless as the rail. And I don't loop over the cue, just keep fingers to the left and right of it. Even less friction.
Probably I lose some important vertical accuracy (tip may hit too high sometimes) but I really feel like I can smack 'em harder this way.
How much speed could you possibly lose on a 20+ mph break, just by having the ball 18 in. further way? And if I really can hit 'em harder, doesn't that offset it? Can I ever get the accuracy of a closed loop bridge breaking off the back rail?