No, you're right that Marrero doesn't have as much power as his slugging percentage implies. He's a gamer, though. He's got a good attitude and, given a little time and seasoning, could become a solid hitter who's good for a lot of doubles and a few stolen bases. .300-15HR-20SB-80RBI could probably be expected of him, assuming he gets to hit in the 3 or 5 slot.
This is technically his 6th season, but he's been brought around slowly by the organization and has been a little injury prone. If he stays healthy and the crazy position-swapping Cards give him his ABs... I think he could be a good solid utility man, or a decent catcher if you don't want to pay the big bucks for a Pudge or a Piazza. Right now, there are easily 5 or more catchers ahead of him, even in NL-only leagues, but I think he's got potential.
Hitting .500 through the first couple weeks of the season has basically just allowed him to get noticed.