I'd put it this way: If you were a patient, would you want a physician who had spent the last 20 years running ships and then passed a medical test, or someone who had spent their career treating patients? And to consider the reverse, would you want your ship commanded by a career physician who had passed a test if there were someone available who had spent their career learning about and progressing toward commanding ships? The US Navy tried a system where medical officers commanded hospital ships early in the 20th century. The idea was, the medical officer was in command of the hospital, which happened to be located on a ship, and the ship itself was run by a contracted merchant crew under a civilian master. Basically the medical officer would decide where to go and what to do, and the merchant captain would get the ship there. In WW1 navy (reserve) crews took over for the civilians, so a naval line officer was in charge of navigating the ship. In 1921 there was a dispute on USS Mercy AH-4 between the medical commanding officer, Cdr William Garton (MC) and the senior line officer, a highly experienced reservist and master mariner, LCdr Athol George. The medical officer wanted George to sign the log with the noon navigation position, but the vessel was in heavy fog in coastal water and George refused because he had his hands full trying to safely conn the ship and didn't know the exact position. Garton ordered him to sign it and he refused again. George was found guilty by court-martial of disobeying an order from a superior. The Secretary of the Navy overturned the verdict on the basis that previous regulations which had allowed medical corps officers to command hospital ships were inconsistent with laws and precedents that staff corps officers couldn't exercise military command over line officers; in effect, Garton's order wasn't a lawful order. Lieutenant Commander George was restored to full duty, and never again would a non-line officer hold a command position in a US Navy vessel.