No protective filters here. Never had a lens front element scratched as far as I recall.
The increase of flare is just too high.
Same for me, but I do use lens hoods to reduce the risk of flare and glare. Because the filter is some distance in front of the front lens element, dirt also shows as being more in focus, and the lens hood is less effective as well because it lacks some extra depth.
The only exceptions are Color Correction filters, Polarizing filters, and ND filters, and under circumstances of extremely nasty dust (if the dirt cannot be rinsed off because it's sticky, or before it dries), like dirt track bike or car racing, or salt water spray, or high up in the mountains a UV filter might be useful. For events like the Holi festival, a rain cover also seems a wise thing to use.
So my take on it is to use the filters for the purpose they are designed for, not mounted permanently. And even when used for special purposes, a multi-coated filter with high-quality plane-parallel glass is preferred.
Cheers,
Bart
