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 large kidney-bean-shaped 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 orbit.

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 efficient transportation system for getting down the hill to Earth -- the only rockets required will be for attitude control and course correction.