What you save using such a thing for launch from Earth is cheap propellant, at the expense of complex infrastructure (including a rail and launching sled capable of carrying an orbital rocket and power systems for running them) and a more complicated vehicle. What makes spaceflight expensive isn't propellant cost, but costs of operating the infrastructure, setting up the vehicle, etc. Adding infrastructure and complexity to save propellant isn't the best approach to making spaceflight cheaper.
On the moon, though, it makes a lot more sense. Maglev technology could reach a sizable fraction of lunar orbital velocity without much trouble. And you can take some shortcuts with low gravity and no atmosphere to worry about...rather than have a long linear track that you dump a huge pulse of power into accelerating the payload along and high speed linear motors to accelerate the payload, have an unpowered circular track that's just passive maglev, or even just rail. Tether the craft to a central motorized rotor, boost it onto the track at enough speed to keep it levitated, and accelerate it with the tether until it's up to the desired velocity. Release when it's going in the desired direction.