Whether and how often the common men sees space marines is kind of irrelevant. Rogue Trader is not a game about common men.

Further I guess I'd like to think my players ego's are not so fragile that if there are bigger and badder individuals out there in the universe their fun is not instantly ruined. My Rogue Trader has a pretty proven track record seeing leadership as more about getting the right people on task on a situation then about personally micromanaging every interaction. Likewise my Arch Militant is more interested in his role as the RT's bodyguard/enforcer (and his own libertine hobbies) then leading the grand army in battle. They already have an NPC as their defacto military forces advisor.

The reason I really like Space Marines is if the players can get them involved they will help tie up any ground conflicts of this scale much more quickly then they would be otherwise. Space Marines are siege breakers, pure and simple, and if the RT gets involved in a military action on the primary planet it will be a siege situation.

The other reason i do like Space Marines is they are one of the few organizations that the RT's have to treat at a purely interaction level. They want nothing that's readily available for sale. Instead the RT has to go interact with them peer to peer and work a deal somehow. That appeals to me.