The same reason the White Sox liked Carlos Quentin, for example. Premium prospects who underperform are sometimes fixable with new/different instruction. This would seem especially applicable in Ackley's case because 1) he was a really good prospect with great peripherals 2) he's only been with one pro organization 3) the mariners developmental prowess is...questionable 4) he has the type of approach the front office likes. It's not a slam dunk, a similar thought process is what brought Ian Stewart to Chicago, but if you're going for undervalued assets(and if the Maholm and Feldman trades are any indication, they are), Ackley is a pretty good choice as a target. For me if we're taking a significant risk on a player it has to be Hultzen. At this point I think it's about 50/50 if he ever sees the majors, but I'd rather gamble on his upside than Ackely personally.