i think they see him as an "upside" player with some raw talent. he basically just replaced taylor on the roster anyway, so it's not surprising he hasn't cracked the rotation yet.
i think the 2012 pick they gave up was lottery-protected though and williams was a lottery pick in 2009, so it seems like he (at least potentially) got better value than he would stand to get with a non-lottery pick in 2012.