I think it can be a real positive. If and when offensive coaches move on and get hired elsewhere, you don't have to keep changing it every couple of years.
Think we are talking different things. The offense is still Nagy's WCO. He still helps gameplan and the playbook and terminology is still all his. The distinction is simply that on gameday the OC calls the plays. If that OC moves on, the offense still remains Nagy's WCO. The playbook and terminology still remains and Nagy continues to be involved in gameplanning. The only difference is a new OC now calls the plays but those plays are still Nagy's.
The benefit of an offensive HC is not exclusively in the playcalling. It is in the fact he will pick and groom assistants that can run his system. So that is what causes the continuity. The playbook and terminology stays.
DeFillipo will not be running his system. He will be running Nagy's system and given the similarities to Pederson (both came up under Reid), what DeFillipo brings to the table is likely some tweaks or a couple plays here and there that maybe Nagy didn't have in his playbook.