Travis Sawchik is right that Stephen Drew would be a good platoon-mate with Jordy Mercer. Drew pounds righties, plays pretty good defense, and would be a big overall upgrade on Clint Barmes.
Here's the problem. In the abstract, Drew might be worth a three-year, $30-40 million type of deal on the open market. But when someone is worth even $10 million a year, he's a good candidate for a qualifying offer, because, as Joel Sherman notes, the team doing the offering then either gets the player on a one-year deal (which is a win, even if you have to pay $14.1 million instead of $10 million to get it, because the first year of any multiyear free-agent deal is the one most likely to be valuable), or you get a draft pick (which is an even bigger win).
Drew is therefore pretty likely to receive a qualifying offer. The Pirates have the No. 27 overall pick, which is unprotected, in next year's draft. Is that worth giving up for Stephen Drew, particularly when Mercer already appears to be a perfectly legitimate big-league shortstop?
The Bucs might well reach the conclusion that it is, since they were willing to give up a competitive-balance pick for Gaby Sanchez. But the pick would certainly factor into their thinking. So they'd have to find a figure for a Drew contract that would both be amenable to Drew (and his agent, Scott Boras) and would give the Pirates a good enough deal that they'd be willing to part with their first-rounder. It wouldn't be impossible, but it would be tricky. Drew might therefore make more sense for a team like the Mets, who have a bigger hole at shortstop (Ruben Tejada was a disaster last year) and a protected first-round pick.