I remember him well from his days as a Tiger. He had a 4 year run there where he was among the top offensive 1B in all of baseball and a real rising star, including his rookie year where he could have been rookie of the year had he not had such competition that season. Then in 2000, injuries hit him and he hasn't been quite the same since. He's shown flashes of the power over the past few years putting up decent totals in limited ABs, so the power he's shown isn't all that surprising. It's the batting average that is astounding. I'm happy that he's finally having another good year. He was a very likable guy.
Why doesn't he play everyday anymore? He's getting up there in age, probably wouldn't hold up health-wise, and their other options at 1B aren't exactly too bad. Chad Tracy has had a decent enough season offensively and spent a portion of the year platooning with Clark, but now the Diamondbacks 1B of the future Conor Jackson is up getting in some occasional work. Despite his great season, Clark may or may not end up backing Jackson up next year.