Disclaimer: I'm only using the corebook and its errata.
I'm going to be GMing my first Rogue Trader game with an entirely virgin (to the setting) group as well, starting very shortly. We were discussing what general character types people might want to play, and our youngest member (also fairly new to RP in general) asked about careers that did less out-of-combat while he was focusing on learning a new system, and the entire table responded "Arch-Militant". He seemed rather happy with that choice and then asked what he'd be in charge of on the ship as well…
I'd assumed he'd be the main gunner and boarding party leader (since you can't fire the guns while engaged in a boarding action anyway, it makes a lot of sense to have one person in charge of both), as the Arch-Militant typically leans on high Ballistic and Weapon Skills and, I had assumed, considered to be a good commander of troops as well. Rereading the ship combat rules, though, I find Weapon skill doesn't matter for the boarding party leader at all, and then looking at the career to help him plan a bit out, I found he doesn't qualify to train Command - the key skill for leading boarding parties - until Rank 3.
Now I'm not sure what to tell him. "You'll shoot the guns, but if we're boarded / boarding, you'll just try to find some way to be helpful"? Aside from possibly picking his Origin Path specifically to give him a ship-useful skill, the Arch-Militant seems incapable of performing ship actions aside from shooting the guns before Rank 3. The only other action I see him taking is possibly Triage, helping to reduce the damage taken from lost boarding exchanges, or Put Your Backs Into It, to help with firefighting and repairs without the normally-necessary skills.
I know if it bothers me this much, I should just change it - but being new to the system and knowing it has a long history and excellent fanbase, I'm hoping for some explanation or better ideas than my "just lower Command and all its upgrades by two ranks for that class" current best solution.