At one time, all services used the same ranks: Captain, Lieutenant, Sergeant, and enlisted man. This is why navy captains are equal to army colonels. Over time, each service developed their structures to suit themselves. Yes, some folks revise that and use one structure. And it works. But it does have a flaw. A ground-pounder with a naval rank feels slighted ("I'm a soldier, dammit, not a squid!"). Sailors with army rank feel less esprit de corps than squids who are actually called by naval rank. I know I'd be pretty ticked if someone told me that I had to be a sergeant now instead of a petty officer!