I actually do agree with the take that Washington increases the Spurs' likelihood of winning a championship this season more than keeping Williams and Beno would. If something happens to Parker or Vaughn, I could see Washington getting minutes in the regular season or the playoffs. Pop's nature is to always play a point guard instead of moving over a shooting guard even if the point guard is a scrub (Jason Hart in 2004 being a prime example).
I don't see a scenario in which Williams would ever play a major role this season. If the Spurs got decimated by injuries, Williams isn't good enough to fill a role in the regular season ... much less the playoffs. I guess there's a chance that he could possibly progress to the point to where he could help, but that isn't this season and probably not next season. In fact, if the Spurs got in a desperate situation, I'd expect them to go after a Keith Langford type player who could come in and be halfway decent. Williams has shown to be unreliable in summer league and preseason, it'd be suicide to rely on him in a game that means anything.
As far as Beno goes, he's pretty much worthless in the playoffs. If a team throws a full court press on him, he becomes the worst point guard in the NBA. The only time you can play him in the playoffs is if the Spurs are facing a horribly inept defensive team or if the Spurs move him to shooting guard.
Bottomline is Washington would increase the odds of the Spurs winning the championship this season. That increase might be like 0.001% increase but he's at least playable if an emergency situation arises. Beno isn't trustworthy in the playoffs and Williams won't be trustworthy on a basketball court for at least two years ... if ever.