[kirrent]

One of a couple of reasons. It's not likely to be as well developed in a review from 81 but an important part of modern high performance sailing is gust and wave response. That is, when you get hit by a gust or a wave, how does your rig and sail flex in response to help you manage. Generally, having the top of your sail twisting off can help with designing for gust response.

[jabl]

Back in the days of square sails, standard practice was that sails higher up on the masts were trimmed looser , in order to give the helmsman time to react when the wind direction changed in a gust (particularly when sailing close-hauled, of course).