I second OM's opinion on zip-offs. I now try to keep my skin covered regardless of the temperature.

In the past three years I have had four basal cell carcinomas removed from different parts of my body. So far no basal cell carcinomas on my legs but I don't want to encourage them there either. Also, I don't like the way that zip-off pants rub and chafe the inside of my legs while walking.

I just wear a pair of lightweight nylon cargo pants such as the REI Saharas but without the zip-off feature.