What's wrong with blocking shots?

Rajon Rondo is 6'1" tall. Marvin Williams is 6'9". Why did Williams decide to hug Rondo instead of trying to block his shot?

It seems to be part of an epidemic that is sweeping the NBA. While Tyson Chandler, Josh Smith, Dwight Howard and assorted other players are still inclined to try to block shots, many of their peers are resigned to just taking fouls rather than challenging shots in an attempt to block them.

Maybe Williams (who claims to be friends with Rondo) knows Rondo struggles at the charity stripe (61.1% this season). Maybe Williams decided that the expected value of Rondo taking him to the rack is more than the expected value of Rondo taking two FT's. Unfortunately, if this was his rationale, I doubt he factored in getting called for a flagrant foul and being ejected.

I haven't talked to Williams (or heard his take on the incident - which looked worse than I think it actually was), but I think he probably thought the safer play was to grab Rondo and try to hold him up than to potentially collide with Rondo in the air. I wouldn't argue with him. But, what would have happened if Williams had challenged the shot?

If Williams had been careful not to foul, Rondo probably would have scored, but Williams might have been able to make the block. If he had gone for the block, there is a reasonable chance he would have made the block and been whistled for the foul. But, with that size advantage, it's not unreasonable to think that he should have been able to prevent Rondo from making the shot.