Sounds like you were being served up a heapin' helpin' of salesman-BS.
Light in front of a filter is just, ah, light in front of a filter, regardless of what happens to the photons after they traverse the rear lens element.
I keep filters on all my lenses. I've used nothing but B+W filters. I can't tell any difference with them on vs. with them off, in a variety of lighting conditions. Just my personal ad hoc testing. I make no scientific claims.
I do know that I've destroyed two filters--one from a near-freak incident (struck by flying rock), the other from blowing sand (where most of us would have agreed on the need for a filter, regardless of our "normal" practice.) So I'm a believer.
I doubt Nikon or Contax would give me a new front lens element for $77!
Exactly my experience.
B + W are of such great quality, that I want to see that BS test that proves it makes a difference on/off.
Plus: for certain lenses it is mandatory to have an UV filter on, or they suck in dust.