That's a bit naive to say. While a filter provides a protective purpose, technically, it changes the optical formula of the lens. Leica, like most manufacturers doesn't design their lenses to accompany a filter. The filter is an addition to the optical design that can change the outcome of the image in various ways depending on the shooting conditions.
Flare, internal reflections and colour variences are just a few of the potential issue that can come with using a filter. Personally, I use high quality filters on my lenses, but then there are times, especially with UV-IR when they 'need' to come off to avoid potential issues.
I could be wrong, but the above info is based on my own experience.