A judgement call
Is it really "necessary?" No, but most boat owners in my yard seem to consider it to be part of basic maintenance. Covering your boat over the winter (via shrink wrap or other means) keeps water off the boat. Keeping water off the boat is good in any weather as it will prolong the life of whatever finish, sealants, equipment etc. you have that is exposed. Covering the boat is especially important over the winter because of the freeze/thaw cycle; water trapped in any little (even microscopic) cranny will freeze and expand, thereby making the cranny bigger and bigger, trapping more water, and so on. Consider: do you keep your car in a garage, or do you wash and wax it more frequently and accept the fact that it will wear more no matter what you do because it is outside 24/7?