I raised this question after Sloan left. The response seemed to be, 1) he wouldn't be traded this year (whoops), and 2) only with an extension commitment from D-Will.
There's a strong argument that Williams would not only make the Cs better, but would be more likely to attract a top FA to the Cs in 2012 (read: Howard). Much as I love Rondo, i believe both are true. I'd agree tho that the risk might be too much without that extension, considering Rondo's strong contract / value considering his game.