Whats the different between just putting the stuff in orbit?
Particularly a useful orbit like maybe a capture orbit so a space mission won't have to waste precious fuel to go get its crap from somewhere else before it continues on with its mission?
Space is not the ocean, you can leave something floating out there and it will stay in a relatively similar spot. Commiting anything to Phobos or Deimos only comits something to an orbit that may not neccessarily be where you want to go and makes it more dificult to get your stuff in the first place (that is touchdown and takeoff from a moon).