The water is cold! If you or one of your crew falls in - and your heavier clothing is going to make you clumsier - hypothermia begins in seconds, rather than minutes. Systems like your fuel pump, that worked happily in the warm summer months might decide that cold, thick diesel fuel was too much for it to put up with any more. Ditto batteries. It would be a shame to go out for a sail and not be able to re-start the engine to get back in before the sun set and it started to get even colder.
Some people do keep their boats in their slips and do go out on nice days during the winter. They tend to be quite careful, and plan thoroughly for any contingency. Your post is a good indication of the careful approach that should be taken to the subject.
Exactly, I thought of those things too.
I have to triple check but I've heard that SeaTow operates during the winter but may have to launch their boat which can take an extra hour or more.