I wear pants to keep cleaner and keep bugs off. Last hike I went on with the scouts. All the other adults and all the kids had shorts. They sprayed down with off head to toe and got redbuds and ticks. I sprayed off on my ankles and didn't get anything on me. Where chicken is going won't have many bugs but maybe ticks. Plus at night I just feel cleaner sleeping if I wore pants during the day.
You heat up fairly quickly hiking elevation with a pack. I think most of the normal trails in those parks are well below 11,000 feet.
Most people wear pants to protect their legs from bugs (ticks in particular) and brush. Whether you need them or not depends on the trial you take. It isn't going to be so cold you would need them i wouldn't think, if that is the line of thinking.
If you are hiking above 10k feet, probably need night protection since I don't think you can build fires over that elevation in that particular area.