If memory serves (and I've misplaced my copy of Fundamentals of Astrodynamics), a zero degree inclination orbit might not be a good idea for interplanetary missions or returning to the moon. An orbit with zero degree inclination would probably not be in line with the plane of the ecliptic, meaning you could end up doing a very energy intensive plane change to launch to an interplanetary trajectory.

If I'm right, the extra mass to orbit gained from launching on the equator could be more than offset by the extra energy it'd take to get on the proper interplanetary trajectory. Of course, it isn't hard to launch into a 28.5 degree inclination orbit from the equator (assuming proper terrain clearance and flight safety issues), but you might not gain enough to make it worthwhile.

As a historical footnote, I recall reading that Jules Verne put the launch site for the moon mission at Cape Canaveral because it was at the best inclination for reaching the moon. As for Mars, I don't know offhand what the best inclination would be, but I'm reasonably sure that putting a vehicle assembly space station in zero inclination wouldn't be the way to go.