Why would you have your elite, all-American edge rusher --a guy that will demand double teams and extra offensive scheming every snap-- play anything other than the position he dominates?
I don't think
@isucy86 or I are saying that he shouldn't be rushing the vast majority of the time. But in a more hybrid position they can use him more flexibly, move him around, and make the offense scheme for him even more, rather than as a strict defensive end playing on one side. For instance, because he demands double teams, if he also drops into coverage sometimes and a third or fourth blitzer comes from somewhere else, that means there are two blockers who were supposed to be dealing with him but are now useless in a way. They either have to identify who to block instead or just wait there in pass protection in case McDonald has a delayed blitz.
There's also the very real question of what the staff can do to best prepare McDonald professionally. If he wants to be an every-down player, he'll probably be doing so as a 3-4 outside linebacker, where he'll need to at least be competent in coverage. Even
Von Miller would drop into coverage, to give the offense different looks and make them be prepared for anything.
