Let me answer this question with a question. When was the last time your best cutter defender spent a scrimmage covering your best handler? How about the other way around? Sure, your best on the ball defender talks all the time about hounding the region's best handler, and your best athlete wants to stop the receiver all others fear, but you don't want to go into a game hoping (or just hoping) your top players match up well against their top players. Funny matchups at practice teach defensive skills and offer insight into what a given player likes or does not like. A handler who loves getting bumped might excel when your grittiest, most physical defender matches up with her. If the tallee cutter/defender you want to try on the handler has never covered someone who gets the disc a lot, then it will be harder for you to change the matchup.