Well, how many MDs would you find practicing in a village of 430 people in general? Or a mine or relatively isolated factory of that size?
Sure, a military vessel might benefit from having a backup for such a key operator. But not everybody can have a backup, or the crew size would balloon. OTOH, the ship can accommodate "A&A officers" and similar drones, suggesting accommodation space or food isn't really at a premium and additional personnel could easily be carried, which makes the existence of the likes of M'Benga plausible. Said existence isn't necessary, though.