The way I feel about heated seat is that even if the 'bottom' as such is not the coldest point of the body, the heated seat contributes to add heat to the blood, and it's circulated around to the entire body. A bit like a central heating system, where the heated seats functions like the furnace.
This. Also, rather than straddle the bike like Michelin man I'd rather hang looser and get some heat in the blood through the bum. For same reason, the best time of year to use sunroofs is in the winter and spring, maybe late autumn, with the heated seats on.