Because there's nothing on the roster that is preventing us from doing so. We know that he can fall back to what he currently is, so why shouldn't we see if there's more there?
I'm not just talking about the power play, I'm talking about better 5v5 time as well.
I've fully been in favor of letting him run with offensive players on the 2nd line to see if there is more there. I've got a number of posts arguing for it.
I think with the PP, given the high scoring situations there needs to be a reason, beyond "lets see if he can do it". If he show's there is more at 5v5, maybe that's the reason. If he improves his faceoffs (which he certainly can), that might be a reason for Evason. Status quo, I see no reason to.
I think you're over-estimating the impact of a 7% difference. And JEE has improved on faceoffs every year he's been in the league. There's no reason he needs to stop now.
The proof is in the pudding. Like I said, look at the players who see PP time, there are plenty there solely for their faceoff ability. Boone Jenner and Jordan Staal are 25 point players these days, and saw 100 minutes of PP time solely to take faceoffs. Koivu, same, Bonino, Bozek, Eller, etc.
It's the reason Bonino will center the 2nd PP unit, and JEE won't get a sniff of it until he shows there is a reason for it. Coaches hate losing PP faceoffs.
