Players find their power stroke. It happens. You act like this is some bizarre occurence. It happened with Youkilis, it happens with plenty of others. They grow into their bodies, they adjust their swings, etc.
Did you read the article about his change in approach this year? Apparently, he had this power stroke in previous years, it was his natural swing the coaching staff thought, and it showed up for batting practice. In games, though, he flatten out his swing and use was he was taught during his baseball upbringing so he could use his speed to get on. The coaching staff has been trying to get him to use his batting practice swing in games for quite some time. Sometimes an adjustment like that takes time.
Furthermore, he's still a star, with his speed making his baserunning and fielding so valuable, even if he isn't a perennial 30+ HR player. The thought all along by the baseball development people was that he'd be a 20+ guy with all those other tools. I'm perfectly fine holding on to that.