The question to whether Roy Williams would make sense in Buffalo depends completely on one person: DONTE WHITNER. If he is the real deal he should be able to handle cover 1's on running downs, and compensate for Williams' lack of coverage ability.
To be honest though, our zones are so soft I can't imagine him being any worse than Bryan Scott, and would certainly be an upgrade in run support.
Like I said, it's up to the coaching staff to evaluate whether Whitner is really a good enough FS to compensate for a liability in coverage at SS. Personally, I say go for it if the price is reasonable. Give Whitner a chance be the player he thinks he is.