Rack and pinion as guides and drive

I posted this on another group and got no answer, maybe ill get some
feedback here.
What would happen if you had a rack and pinion drive, and you just
support the gantry or bridge on the pinions, instead of using linear
guides, just use the pinions as traction wheels. some other "idlers"
should be used to prevent the bridge from moving sideways, and so it
wont rotate on its axis.
What do you guys think about this? any coments, suggestions are
welcomed.
Best regards
Fernando