My dad's on blood thinners that make him VERY cold. Like, we were having a heat wave in July in NY, and he STILL kept the heat on. Meanwhile, I can overheat and pass out with very little notice whenever the temp goes too high. When we drive anywhere, I have to have the air conditioning blowing on me, and he has heat blowing on him. It's still not enough, without the cold air blowing up through my seat, and the hot air blowing up through his. That keeps both of us functioning, without either of us being miserable.

But, yeah, in most instances, it's not necessary. :-) If you don't like cold leather seats in the winter, or hot leather seats in the summer, get cloth seats. :-D