Rowland allowed an amazing 15 homers in 68 innings in the Pioneer League last season, but he struck out 52 batters and walked only 17, and he was a third-round pick in 2010. He's also 6-foot-6 and just turned 20 in December. So he might have a bit of upside as a player with a decent pedigree and some projectability. The reports I've seen online have Rowland's velocity anywhere from 89-94 MPH, so I suppose we'll just have to see, but it doesn't sound like his velocity is overwhelming right now. The Bucs must feel like they're getting the higher-upside player here, and this is a spot where you have to just trust them. Even if they're wrong, it arguably doesn't matter much.
The Pirates also released minor-league reliever Michael Dubee, who had good minor-league stats but never even got much of a shot at the Class AAA level and probably didn't have the stuff to be successful in the big leagues.
Update (by Vlad): A little more info on Rowland. He's the son of former ML catcher Rich Rowland, and in high school he drew a significant amount of attention from D-1 schools as a basketball recruit. He was more interested in baseball, however, and signed a letter of intent with Oregon after putting up a 0.24 ERA and 105/10 K/BB in 58 innings as a high school senior, in a season that included two no-hitters, one of them a perfect game. Baseball America rated him as the #71 prospect going into the 2010 draft (and the #12 from the state of California), citing his athleticism, bloodlines, and physical projection, and he signed for $395k. His early results in pro ball were obviously very poor, but he's a bit more interesting than the average struggling pitcher in the low minors. This article claims that he made significant alterations to his mechanics last year, going from a straight-over-the-top delivery to more of a three-quarters arm angle, sacrificing some velocity in an effort to get more movement on his fastball. It'll be interesting to see whether the Pirates' coaches have him continue in that direction, or whether they have him work in something closer to his original style.