It's not just a defensive issue with Williams playing the 3 spot, imo. His handles are suspect for an NBA sf .. too many turnovers and too few assists (at the college level playing PF to assume his offense will translate well to the SF in the pros. He may be great, in fact, I think he will be, I just think he's a stretch 4 and entirely redundant on this team.
While I agree with your concerns, especially on defense. His handle doesn't bother me at all. How much do the Jazz ask their SF to really handle the ball? As long as he can hit the 3, post up smaller players, can finish around the basket, and drive to the rim he can play the 3 in the Jazz system.