The trade-off? "They do not experience the jitters and inhibitions the rest of us feel." Talk about a huge advantage.
I suspect that (seemingly) ugly tendencies like confirmation bias, stereotyping, vengeance, shame, etc. all increase your personal odds of reproduction. Likely at the expense of society, but it is what it is.
I can understand how complete social trust and lack of anxiety would be beneficial in a perfect world; alas, we do not live in one.
http://lesswrong.com/lw/l0/adaptationexecuters_not_fitnessma...
The disease comes with a large number of physiological problems, and also serious problems in intelligence outside of social behavior.
The boundaries between races are, after all, determined by culture (a hundred years ago, Italians, Russians, and Jews would not have been considered “white”), and culture has to be learned. To make matters worse, Williams is associated with severe deficits in visual perception (http://www.udel.edu/PR/UDaily/2004/WSDrawingslg.jpg), so it’s possible that someone with the syndrome would have trouble even recognizing the facial characteristics that neurotypical folks associate with various races.
As others have mentioned, the condition also results in physical changes including pale skin, stunted growth, and abnormal facial features. It's not something one might wish upon one's children.