Dietrich-Smith is an example that a great quarterback can make an offensive lineman look really good in the passing game, and a bad quarterback can have the opposite effect.
That's about where I'd rank Smith, too. He wasn't exactly good last yer, but he was put in a pretty terrible situation and has a history of being a much better player. He's no Pro Bowler, not even as a backup, but he's an average center who will do what he needs to do, but won't blow anyone off the ball. Essentially, Smith was what the Bucs had in Jeremy Zuttah before they decided they need to replace him, for some reason.
A lot of Bucs fans don't seem to agree, though. I'm not entirely sure why, but Smith rapidly gained a reputation for being terrible. He certainly wasn't living up to the contract the team gave him, but he wasn't that bad -- certainly they couldn't have found a better center this offseason.
Besides, it's not like the Bucs are about to replace their center while replacing their left tackle and right guard, too. They tried that last year. It didn't really work out.