Dunno if the roller saddles would actually move when using the Bigsby…I've always figured the point was to give the strings a smooth surface to slide back & forth on. The string spacing adjustment is a handy feature in any case.
-Dave-
I'm guessing they probably wouldn't move either, otherwise they would of used a different bridge. If they did move, I could imagine the string spacing ending up very random, as you would probably find that some rollers would not return to their original starting position and could drift a fair bit over time, maybe even coming off the screw