backwards riding is useful when you start to get out into an intersection and then realize that you don't have it -- you can go backwards a bit. but unless you are riding a polo bike, half a crank rotation is generally enough.
if you can skid into a trackstand, then backpedal just a little bit, then you've got all you really need. once you've done this (useful) thing for a while, you might try ****ing around doing (useless) backwards circles sometime. I did my first one ever on 2nds ave at 23rd street once, after doing aforementioned halfway jawn. I had room and I decided to keep going for ****s and giggles.