Originally posted by krissow But would a cheap filter like this worsen my image quality?
Yes.
If I am Pentax (or Leitz, or Zeiss, or . . .) and I am careful to design a lens that corrects for all kinds of aberrations, lens flare, etc., and I charge accordingly for that lens, does it make sense for my customer to slap one last (critical) piece of cheap glass on the front of the lens and hope for the best?
I do not even use "protective" filters because of the ghosting I get with them, at least with wide angle lenses. I have found that using a good lens hood (and just being careful) provides for enough protection. But whether to use / not use such a filter seems to be a religious thing for a lot of people. If I made the decision that I just really HAD to have such a filter, I would at least buy a good one, like a B+W MRC -- and only from a reputable dealer like B&H or Adorama (eBay and Amazon are full of counterfeits).