One of the uses of a mass driver could be to send payloads directly to
L4 or L5. Of course, the L4 and L5 points are not really points, but rather
orbits that move ahead or behind the moon. The shape of the orbit is mostly due
to the inclination of the moon's orbit to the ecliptic. However, we could
probably hit L4 or L5 by fine-tuning the launch velocity.
It is unclear what advantage launching to the Lagrange points provide,
however. Assuming the payload is bound for somewhere besides L4 or L5,
we'd have about the same delta-V to get there as we would from low lunar
If we go for low lunar orbit, we can loiter until precession gives us a more
favorable azimuth. However, it means we can't use the mass driver to throw
things into a Mars trajectory except when our mass driver is pointed in the
right direction; and that means we're back to paying the rocket equation
for the big boost.
Launches directly to Earth orbit from the moon would be less of a problem.
Earth will wander in the sky (plus or minus 7 degrees, north and south) but a
small course correction burn early in the flight should take out that much
error. If we can make aerobrakes from lunar materials, we'll have a very
transportation system for getting down the hill to Earth -- the only rockets
required will be for attitude control and course correction.