I would not, under any cir stances, sign Marcus Williams to a contract at this point. Hes a d-league stud but does not have the physical skill-set to stick in the NBA. I'd rather they look at a player like Hairston or a big like Sims. Williams has had 2 full Spurs contracts already. I see no reason that a 3rd would yield different results.
What I have heard is that Marcus does NOT have the lateral quickness to defend NBA caliber wing players. He's essentially a very good d-league combo forward, lacking the skills to be an NBA wing and the size to play with the NBA big boys in the paint. In baseball terms, he's Triple-A for life. He'll probably get a few call ups over the next few years, but the only way a player like this ever sticks is through massively hard work, and that's never been his M.O. It requires a massive mental shift to go from thinking "I'm a star" which he was told his whole HS and college career, to "If I work very hard, I may stick for a few years in a 9-15 bench role". I don't think Marcus has that in him.