I agree on the non automatic, but having some form of life jacket on can be quite important. During this years Chesapeake Bay Schooner Race one man was lost overboard while dropping sail. I don't know if he was conscious when he went in the water, but having one on may have saved his life. Note he was a highly experienced sailor.

http://www.wavy.com/dpp/news/local_news/crewman-dies-after-falling-off-sailboat