It'll be a very long time before the facilities and personnel exist on Mars to build spacecraft. For that, you need:

1. The ability to produce very high quality metal alloys and composite materials,
2. the machinery and skilled personnel to shape those materials to extremely tight tolerances (+- 0.001 inch or less),
3. the ability to build avionics systems (integrated circuits, circuit boards, related hardware, and software),
4. the ability to build and maintain the systems (structural, thermal, environmental, electrical, etc) that make up spacecraft, and
5. the ability to train crews and house them between missions.

Some of the machinery needed to do these things weighs many tons and won't be easily transportable to another planet. It takes the skilled efforts of a lot of people to build even an ordinary commsat. More advanced vehicles like manned spacecraft or space probes are much more difficult.

Until we can do those things on Mars, the only alternative is to do what we're doing today - build them on Earth where the infrastructure exists. That's why it makes absolutely no sense to think we can do this stuff from Mars for the foreseeable future. LEO is the gateway for launching not only space probes going towards or away from the sun but also for satellites intended for high Earth orbit.

A refuelable space tug has the potential to radically lower launch costs, especially for countries like Russia that launch from high latitudes. For example, a Proton booster can put up to 20 metric tons into a 51 degree inclination LEO. However, to send something to geosynch from that inclination requires an upper stage that currently weighs about 18 metric tons, leaving only 2 metric tons for payload. Instead of spending over $60 million on a Proton to put 2 metric tons into geosynch, they could use a much smaller and less expensive booster to put the same payload into LEO and contract for a tug to take it the rest of the way. That same tug could have the ability to accelerate space probes to escape velocity, which would allow for heavier probes being launched on vehicles like the Delta II than is currently possible. Depending on the number of Castor strap-ons, a Delta II can put up to 6 metric tons into LEO. It can only lift about a third that amount to GTO, and even less on an interplanetary trajectory.