My buddy's opinion was that since a shot has already been taken, the rack has been accepted as-is and should not be touched in a subsequent inning.I guess that's the rule but I don't see why. I could see not specifying the rack could be used, but forbidding it?
My thought is that it may be a matter of consistency. In the case where the 15th ball needs to be spotted, but the cue ball interferes with the rack, you would not be able to use the rack so they prohibit its use in all cases. I have never seen this come up in play, and in practice we do use the rack when spotting the 15th ball where the 14 ball rack has not been touched.