it does make sense to have a spacestation as a jump point for the moon, then have moon as a base to mars... but having a moon base and also a space station means two bases to maintain. twice the amount of supplies need to be delivered, and shipping and handling costs are pretty expensive for these guys.
Also for the rocket or space shuttle to get to the space station they need fuel, then move onto moon, they will need more fuel. but if they go directly to the moon, they can use some of the velocity they picked up escaping earth and might save fuel and time.
i know you are probably thinking having shuttle service only from moon to ISS or something. but the problem is that ISS or any other space station can't support the moon's supplies since the ISS can't hold much supplies itself. and if going to be used for such a role will need huge investments and upgrades that no country building the ISS is willing to support right now.