I guess I'll toss in a contrary opinion. I wear zip-off convertible pants during summer hikes and I do zip them off for a couple of hours a day, mostly on uphill pulls in the late morning or afternoon.

I carefully ration my sun exposure and I far prefer to use clothes over sunscreen lotions (or deet for that matter), but the ability to ventilate my legs on the sweatiest trail sections during hot days is valuable to me. I even partially unzip my legs at times so they ventilate better, but still cover my skin. ymmv