He's not strong enough to deal with a lot of post players, not just Dwight Howard. He's a solid pick and roll defender, but is a bad weakside helper. He's also very foul prone, which is okay but if Shaq goes down you're in serious trouble
While I agree that he plays the offensive role of a 5 just fine you don't want him as your only backup C to a 37 year old C.
If they trade Z its a huge gamble that both Shaq is going to be healthy all year, and that Hickson can handle big time minutes during playoffs or big games. Either that or LeBron will have to go to the 4 full time. I don't think that's the best strategy for the Cavs though. He has a lot of advantages against most 4s, but matchup wise it'd hurt the team if they do it more than 12 minutes a game.
You need size and length to deal with the three of the four other best teams in the league. (Boston, Orlando, Lakers) Cleveland trading Big Z will put them at a big disadvantage in the playoffs because of that.