I don't think teams shade their CF much, if at all, based on who is playing either corner. They shade their CF based on who the hitter is, and who the pitcher is.
Other good hitters get "pitched around" in scoring situations too, yet they find ways to do damage and drive in runs. I believe his SLG/ISO is part of the explaination, a too passive approach another part.
It depends which metric you chose. According to Fangraphs, 2015: By DRS, tied for last among 24 RFers By UZR, 10th of 24 By "DEF" (a rather undefined combination of factors), 10th of 24, right between Heyward and Harper.
I don't think it's pointless...it matters. Bernier was a waste of a roster spot, but ultimately had zero effect. Stauffer will pitch in winnable games. there's a good chance that has an effect.