My coop is small. I have both metal window screen and hardware cloth on the openings of my coop and run. I did it to keep out the mosquitoes. It has worked good so far for that purpose. It keeps out flies too.
I live next to a pond and down the road from the San Gabriel river. We get LOTS of mosquitoes in the winter and spring. There have been a few cases of West Nile and a couple of Encephalitis/Meningitis in people within a few blocks of my house, and many dead birds a few years ago. The screening was an added protection. The screening does get dusty after awhile, but since my coop is small, I just go over it with a broom, and in less than few minutes it is good as new. It would never keep out the raccoons, so that is why it is a double layer with hardware cloth.