I live in FL, so i'll comment on this one. Here, 95%+ of pools are enclosed. FL has some nasty tropical insects that you don't see out west in TX or CA due to the humidity. That along with the gators and snakes you probably have near your parts as well.
However, the screen enclosure helps *greatly* with the cleanliness of the pool. Dirt, lawn clippings, tree leaves, cones, etc won't make their way in the pool, or onto the deck. This is also generally why even larger pools in FL are built with only one skimmer, instead of two. There simply isn't that much debris to worry about a skimmer catching if the wind is blowing one way, or the other.
Screen enclosures also had significant cost to the project as well. In the case of the pool I was pricing, a 17x35 geometric w/ attached 7'octagonal spa and a 6' perimeter paver decking around the pool, the screen enclosure added $9100 to the total estimate.