i am not a lens designer, but the best I can tell, UV filters do nothing for color film. Atmospheric haze is usually visible in the normal spectrum in the form of dust, smog, and mist. I have never seen a "with uv filter/without" example that was not doctored.
It may be that pentax lenses have a coating that blocks UV light. But I dont think any lens really needs a UV filter unless you are using old-school film that may be oversensitive to UV.