I was in Reno over the weekend and watched a couple of hours of the ladies bowling the USBC Womens Open. It seemed like 1 of every 4 or 5 bowlers were using a back-up ball of some degree. But none with much success.
I did strike me that it could be a good option if you could master both styles. When the right side of the lane breaks down, just move left and you have a fresh pocket! LOL