Consider the points made by FatBaxter in post #20: type of heat and type of fairing can make HUGE differences in what does or does not work.
I've only ridden in the "dry" heat of the west and most of it behind a modest RS fairing. When I tried riding in jeans on hot days, my legs got fried - by engine heat off my K bikes. (Think about what happens when you stand close to a campfire in jeans!)
My legs have been comfortable in the "low tech" combo of jockey underwear, light cotton blend trousers and leather pants from moderate temps up to about 100F.
When I bought my Rev'It Sand suit, I tried removing the rain liner from the pants, opening the vents, and wearing shorts over the underware. OK, but think I think less comfortable than with the long pants and rain liner in place. I'm guessing the air space between those three layers insulate my legs from both engine heat and really high ambient heat. Anyway, for me it is one less thing to think about.
We're all different. I can't remember ever really noticing hot legs (with the combo's mentioned) or for that matter hot hands (never had vented gloves) or hot feet.
Dealing with cold EVERYTHING is another matter which has been covered pretty well.
I also live in the Philadelphia area and ride with my BMW Airflow suit once the temperature gets above 80'. I'm not a fan of overpants (too bulky) for me.