Being a percussion guy myself, I can say from experience that the Dr. Beat is entirely necessary. I haven't marched corps yet, but I've marched WGI, and we use it the whole season. Even the best drummers can't maintain the correct tempo 100% of the time, so the Dr. is there to help with that. It certainly helps build consistency with tempo. I'd say it wouldn't be a good idea to stop using it at any point. Maybe less later in the season, but never all together.
I'd file it under the "necessary evil" category, although I don't really find it evil, heh.