Pros and Cons of shoulder rests

A few months ago my son stopped using a shoulder rest. (Naturally he made 
this decision shortly after I purchased a new Kuhn to replace the decrepit

one that was marking the varnish on the back of his violin....) His 
teacher--who studied at Julliard and the like--had a teacher way back when

who forcibly took his shoulder rest away, and he plays without one. He
told 
my S this story, whci is apparently what triggered the experiment, which
now 
seems to be a permanent arrangement.

I was wondering whether people here play with one or without one, or
switch 
back and forth, and if so, why?