Think of it less as the person and more about the operation. We need men flying outside the bounds of our borders to search for this last artifacts of ages past, or whole lost imperial worlds.
It also grants us an opportunity to honor a person for their service, without losing someone Too valuable that are more needed Here. We choose the men that've had to have been passed up for promotion, or shown a certain lack in something we need in our highest officers.
I daresay even in your case, some Astartes may be close to Company Captains, but you have your Captains already. It's highly irregular to grant a Space Marine a Warrant, but, this could possibly be a large boon, think of the publicity, and with a lesser chance of them turning against us. Perhaps we should consider the Salamanders or some of their branched off chapters, they have families that can continue that Dynasty should the Astarte ever fall, or are especially needed.
I Dont know, how could he have possibly made it past the checkpoints if h-
Master of Assassins, is that you? You got my beard wrong, i trimmed it mid transit here. Your intel wasnt as good as you thought, maybe the plants you had aboard my vessel were, Caught?