If you do any serious hiking, I think gaiters are a necessity. I wore a pair of light weight Altras over my trail runners when I hiked the CDT (3,000 miles) last year. They went a long way to keeping the dust, rocks, sticks, etc., from working their way into my shoes and irritating my feet. I think they're a way to help prevent blisters.
I also like to wear my Kenetrek gaiters over my Kenetreks while bushwacking and stream crossing. Again, the whole point is to keep debris and water out of my boots. Happy feet make for much happier hiking.