I wouldn't say that rapping on a 58 and a 60 is unsafe if handled properly, but Dru's point should suggest it is a hassle on routes with multiple raps when one would like to thread the rope you are pulling through the anchor as you pull. If you are using a tag line or some other method that doesn't allow this, so be it. But if I'm rapping on doubles or twins, I want to be able to alternate which rope gets pulled without thinking about one rope being longer than the other. Of course this can be dealt with, and YMMV, but in my climbing world I just don't see any big win by hanging on to that extra 2m of rope on the longer rope. I would have evened up those ropes as soon as I could.