Shaq could have a huge impact on the Cavs. He's another player they can pass the ball to, so they don't give it to LeBron every time. Even at this stage of his career, he's a big improvement over Ilgauskas.
Williams playoff performance was terrible, he wasn't a strong enough player to command defensive attention. I expect the Cavs aren't done and will look to add someone else to score along with LeBron and Shaq.
If they could get Hedo....