A lot depends on the terms, and there's a lot of risk here, in that Tabata is still relatively unproven. But this is a bold and proactive move that could end up looking very smart for the Pirates. Without a new contract, Tabata would have been eligible for free agency after the 2016 season, at which point he would be entering his age 28 season. If he develops as the Pirates hope he will, he would cost a ton on the open market. If Tabata's contract includes three option years, though, the Bucs will control him through his age 31 season.
Tabata, meanwhile, is currently making close to the major-league minimum salary. This deal would set him up for life, and for that reason, the Pirates hopefully won't have to pay near market rates.
Tim Dierkes of MLB Trade Rumors has been the one tweeting much of the information here. We'll have more updates on this situation as it develops. In the meantime, head over to Bucs Dugout for more updates.