In the context of a street performance, the difference between a bad musician and a good musician is much greater than that between a good and a world-class one, particularly if you're only briefly exposed to it. For the most part, you're just not focusing on the music - it's not like sitting in a hall intending to be entertained.

Should people at a metro station even be able to tell the difference between a violin worth $3.5k and one worth $3.5M anyway?