That's cherry picking the data. Going back the last five years, he was injured the first two, and recovering from shoulder surgery the third.
Still, in those first two injury years he scored 22 points in 22 games and 42 points in 47 games. Since then, he's taken the back seat to our shiny new toys, Eberle and Yakupov. So while you may look at the minutes he plays and think he's getting the ice-time, consider that he's on a line with Boyd Gordon and Matt Hendricks, who you should be familiar with. Starting most shifts in the defensive zone against top opposition. Those aren't quality minutes for putting up points. On the worst team in the league.
Regardless, over the last five years he's averaged .632 PPG, which is just over 50 points a year, pro rated. Even if you take only the last two years, a relatively healthy Hemsky still puts up .489 PPG from the third line, playing with grinders, against top lines.
So to put it this way, at 2.5 years older than Stalberg, Hemsky still puts up more points. Even at his lowest standard, he's going to score more. I'd argue that with a fresh start, he returns to around the .6 PPG mark, not quite as high as his career average of .732, but an improvement on his current pace. Stalberg on the other hand, hasn't proven that he's going to be more than a 30-40 point guy. If he hasn't made a breakthrough by now, he's not likely to. So I think a pretty good reason to believe that a guy can be productive, is if he's done it before.
As far as the contract goes, it expires at the end of the year and he's only owed another 1.475 for the remaining games. If he helps push you towards the playoffs, that's not even a consideration. But having said that, Edmonton will likely retain salary IF the return is right.
I think he's going to be the steal of the deadline for whoever gets him. All it costs is magic beans as well.