Elon Musk has said from the beginning of SpaceX that trips to Mars would have to return as well. It's not because of the humans but because the cost of producing the space ships will be so large that it's not feasible to shoot them to Mars once and leave them there; they have to be reusable.
> The principle behind all of its vehicles is total re-usability; every single system must be able to be serviced and pressed back into operation. This is to cut down on the already astronomical cost of space flight (pun intended), making it more affordable for private companies to conduct missions into the heavens. The company’s main goal is to eliminate the equipment cost for space travel, leaving fuel as the only financial burden.
> The principle behind all of its vehicles is total re-usability; every single system must be able to be serviced and pressed back into operation. This is to cut down on the already astronomical cost of space flight (pun intended), making it more affordable for private companies to conduct missions into the heavens. The company’s main goal is to eliminate the equipment cost for space travel, leaving fuel as the only financial burden.
http://www.extremetech.com/extreme/122923-spacex-ceo-claims-...