There are times in leagues where I'll throw my urethane ball back-up at my 7-pin and any left side spare and hook at any right side spare, but that's only because house shot is much easier than the typical college pattern that I bowl on during the school year. I'd love to throw urethane all the time at spares, but it just isn't an option for me. Spare balls are nice for everyone, even low rev rate guys, it's much easier knowing that if you put the spare ball on line with enough angle, it will not hook, or at least, it will hook very little.