I suppose the need for screens depends on the area you live in. I haven't really had a problem with things coming in through the windows where I live, so from my experience, I haven't felt the need for screens here - per year, we get maybe 2 or 3 bees/wasps, a few spiders/flies and the odd moth if we leave lights on at night - which, while an annoyance, are not frequent enough occurences to warrant screens. In the states though, I'm all for screens when there are mosquitos and other unwanted creatures around; similarly if I was living in an area of the UK which, as groovy_yank and balmerhon have mentioned, has lots of insects and flies lurking outside the windows.