The thing with Sprong on LW is that if they viewed Sprong as a long term LWer, they would have moved him to LW by now. It makes no sense from a roster perspective or looking at Sprong's strengths for him to not be playing LW, but they've made no indication that they want to shift him over to LW. If they were even considering it a possibility, I think they would have had Sprong at LW by now. Not only in the NHL, but also in the AHL.
I think the reality of it is that they don't trust his play away from the puck at all, to the point where they don't even want to gamble on putting him on his off-wing. It doesn't matter that LW is a much bigger organization need, he'd be getting much better chances on LW and LW fits his style much better, they seem so concerned about his play away from the puck that they won't even consider it an option. It's stupid, but it is what it is.