If Spurs trade for Williams, they had to be high on him because there is an important financial commitment coming with him. His salary this year is $4.8M, $5M for next season and $6.3M for 2014/2015 with the deadline to pick the option on that year being at the end of next season training camp.
Personally, I would have no issue at all with Spurs trading both Blair and Bonner if they can get a good PF. As you said, Blair could/would be a pain in the ass to have at the end of the bench. I consider Bonner being worthless, even as a 5th big, because, at the end, it's all about the playoffs and he is atrocious at that time of the year.