A screened in porch is a preference type thing in this area. Many new homes in the upper price range are going with outdoor kitchens as the new rage - in that case a screened in porch doesn't fit the bill, unless you install an elaborate exhust system. Homes next to ponds or water sources (canals, rivers, small lakes) seem to fair well with screened in areas because of mosquitoes, gnatts, etc.
As far as adding value - do it right - give your patio the WOW factor and you will get every penny (maybe more) back when it is time to sell......plus you will have a feature that you can enjoy!
Hope you have a happy Home-warming & best always,