Trading Rondo for him is a terrible idea. Even in the best case scenario, he isn't good enough to single-handedly carry the C's, which he'd essentially have to do.
I'd be intrigued about acquiring him cheaply to pair with Rondo. Williams could play some 2, and obviously they'd be a matchup nightmare. It'd be a risk, but a risk that would leave them potentially one star away from contending if things worked out.
The problem is that the Nets would have to blow it up to make a deal like that, and given that they've traded away all of their picks (or given team the right to swap with them--mostly us), I don't see the Nets doing that.