It is a blanket statement to say that most street musicians are awful, but in my experience most buskers can play one or two tunes badly, simply using their instruments to get cash from the tourists.
But not all; in Krakow they have a great system. There are specific places where the musicians are allowed to play and they have a roster. Every hour or so all the musicians get up and move away, to be shortly replaces by another musician. They are also (mostly) very good.
Once such muso was this guy. He was playing classical guitar; most of which he was improvising on the spot. He was exceptionally good; clearly classically trained. We ended up sitting for quite a while listening to his guitar singing.
The system is Krakow is great (Prague has a similar setup). It lets you can sit on Krakow Square and enjoy the music without being harassed by buskers. Well except for the unofficial buskers that disappear at the vaguest sign of authority.