I think you and all of us really are spoiled by having Beni, Bradley, and Betts--all fast with pretty good gloves and two with good arms-- out there, which historically is abby normal for the Red Sox, especially in LF. At the same time, we also want the good hitting, which last year we had from all three, especially MVP Betts.
What Cora did last year and again this year was follow that old Sox tradition and sometimes give up some defense for ensuring good bats in the belfrey/outfield. Thus JDM has played both LF and RF. He ain't no gazelle, that's for sure, but he has a good arm and rarely misplays the balls hit to him. Moreover, he likes to play the outfield. I can live with that and apparently so can Cora and DD.
The problem this year, however, is that no one is remotely at good batting DH as JDM is. I think Mookie did it once and did well. Otherwise, no.
One more reason to use JDM out there is that Cora has had to keep 13 pitchers on the roster, which means just 3 subs for the rest--1 catcher and 2 infielders, with JDM doubling as the 4th outfielder.