i would think that having one on board for emergencies might be nice,
like if you fall into the water in the north atlantic and get
hypothermia really bad and need to get warmed up or something. i
remember from my sailing class that the instructor said that when your
body gets down to a certain temperature it can kind of maintain it's
temperature but that it doesn't produce enough heat to warm itself back
up, even with blankets and things piled up on it. so you have to have
some kind of external heat, from another person wrapping around you, an
electric blanket, a fire, etc, to warm back up. ok, so that doesn't
make sense to me, but that's what he said. i would imagine that no
matter how little heat you were producing if you covered up with enough
blankets eventually you'd have to get warm, but then if i had a license
to practice medicine it would have to be out of a cracker jack box,
what do i know about it.

but other than for hypothermia, i wouldn't bother. i was kind of cold
this winter so i tossed a couple of thin cheapie 100% cotton blankets
from walmart on top of my bed and i'm telling you it sure heated things
up. even when it's really cold i usually have to fold one or two of
them back or it's just too hot to sleep at night.