I think the hard line against filters, with the defacto answer "they reduce quality", is a bit of a misnomer. I generally don't use screw-on filters, however I do use the Lee filter system with solid and graduated ND filters. I've done shots with as many as four filters (some glass, some resin) stacked on top of each other to properly balance contrast in a scene and get a proper shot.
I often get responses like "You should just use HDR", and "Stacking filters makes flare worse", etc. Sure, both of those things are possibilities, however they are not guarantees.
I could bracket my shots and merge to HDR, however that is ultimately doing the same thing as my filters, and its often a LOT more work. HDR often produces its own artifacts as well, such as halos, funky tone mapping (i.e. blacks end up lighter than shadows, creating that oddball "classic" HDR look you see all over the web, and it can take a LOT of tweaking to get the darker tones looking ideal), and strange grades or flats in highlights.
It is also possible that my extra filters will make flare worse...but that assumes I'm encountering flare at all. An extra piece of glass in front of a lens can compound problems with flare since its another thing that light can reflect off of, however it is also easy enough to block non-incident light. You can use a lens hood (with screw-on filters), attach a post-filter lens hood with something like the Lee Foundation Kit, or in those cases where a hood doesn't solve the problem, your own hand often does the job when held out at the right angle.
Bottom line, those who take a hard line against filters do have valid arguments, assuming you run into the problems they are proclaiming will destroy your shots. There are many ways to control those problems, however, and in the grand scheme of things, the visual quality of a shot taken with a filter vs. without is usually pretty minor at in the average case, and non-existent in the best case. As such, I don't care much for the defacto response "Don't ever use filters ever", and I prefer to answer on a case-by-case basis. Sometimes avoiding filters is the right decision (i.e. your shooting a lot in near-direct sunlight, which can pose a big problem with flare), and sometimes using filters can solve real-world problems. Tune the answer to the question.
I should also mention that there is a certain appeal to working your light on-scene. ND and GND filters are contrast balancers, and do the same job as digital grad filters or HDR: limit light and compress dynamic range. They just do it in analog form. I can't say how much more I enjoy landscape photography now that I am able to fully craft my entire shot on-location, with my camera and vision, rather than have to craft half of it on my computer once I get home. I rarely do much in the way of post-processing these days, and use filters and manual exposure adjustment to create my photographs while I take them. Its a great feeling, and an experience I wouldn't have if I had listened to all the anti-filter nay-sayers.
