Why would having an IT insert in your spare ball require a second set of inserts? Most bowlers I've seen will just pop the thumb out of the strike ball, insert it in the spare ball and then shoot the spare.
As for having one break in a strike ball, yes you would have other balls to use, but in the modern game, especially in professional tournaments, having the right ball is very important. While you might be able to get one of the other balls to hit the pocket consistently, you might also wind up consisently leaving the 10 pin or the seven pin.
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