Personally I dislike any type of long pants on the trail or in the city but I wear them when temps/bugs/flora/abrasions dictate.

I don't wear zipoffs to save weight. I wear them for convenience. When the day starts out cold and then warms up, or when I start on a trail and end up bushwhacking. I don't like them for long trips in the summer because, after wearing shorts all day, I like to have a relatively clean pair of long pants to wear in camp when the temps drop. In general I use zip offs for dayhikes and shorts with separate long pants on overnights. In my case, both systems weigh about the same.

One thing to watch out for on zipoffs. The zippered seams between the shorts and the pant legs can chafe your legs after many hours. The zippers are usually covered with a small flap of fabric on both sides which makes the whole bottom edge of the shorts rather bulky. Like anything else, YMMV.