It's their job. You could say that if you want to hear how a violin really sounds, a good violinist is the worst person to give it to. Good violinists impose their own sound on an instrument. They tone down harsh notes, smooth out unevenness, and like a chef using salad dressing, blend everything together with vibrato. What you hear is not so much the sound of a particular instrument as the sound of violin-playing, which is a kind of magic trick they do, using the violin as a prop.
Bad violinists are even less useful.
More seriously, when someone plays a violin, there is an unconscious tendency to adjust to the instrument. This makes it difficult to evaluate an instrument independently of a given player's response to it. Of course, there is a lot to be learned from talking to violinists. They tell you things you don't hear while they are playing, such as how much or how little work it took to make an instrument sound good. And this is, after all, a major difference between good and bad instruments.