Sanchez is 28 and is in the midst of a thoroughly horrible season for the Marlins. I don't mind losing Hernandez, obviously, but giving up a draft pick for a player like this would be a little weird, especially since the Pirates have a perfectly-functional first-base platoon, especially now that the Travis Snider deal will help keep Casey McGehee on the bench against righties. Sanchez has hit lefties well in his career, but McGehee can hit them too. McGehee really is fine if you just use him against lefties, and he has an .807 OPS against them this year, so even as an upgrade for this year, Sanchez isn't much.
Sanchez had a couple reasonable seasons with the Marlins in 2010 and 2011, so maybe the Pirates are hoping he'll bounce back. But I think a good result here would be that the Pirates would get an average first baseman for a couple years, and one whose salaries will rise due to arbitration. I don't see a whole lot of upside here, and I'd prefer to have the draft pick.
UPDATE 3:08 p.m.: The deal is done.
UPDATE 3:27 p.m.: The Pirates also evidently got minor-league reliever Kyle Kaminska in the deal. He's a big 23-year-old righty who has posted nice strikeout and walk numbers. He's struggled with home runs this year, but has a good groundball/flyball ratio and hasn't had that problem in the past, so that might be a blip. I can't find much about him as a prospect, since nobody cares about the Marlins.
UPDATE 3:35 p.m.: The deal is official. It's Sanchez and Kaminska for Hernandez and the competitive-balance pick.
UPDATE (BY Vlad) 4:43 p.m.: There's some more information about Kaminska in this trade writeup by Baseball America. On the whole, he doesn't sound like anything special, if they're to be believed.