Similar to the idea of using lasers to propel solar sails.

The beauty of using lasers is that there is a technique (I think invented by Robert Forward) where the laser-sail spacecraft ejects a large part of the sail and turns around, keeping a smaller chunk of sail for itself. The laser bounces off the large sail section, and hits the reversed spacecraft's smaller sail, slowing it down. The large section, of course, is accelerated into the great beyond eventually. Don't know if the focusing requirements of this are feasible, but in theory it would work.

Don't think you can pull a trick like this with a mag beam, unless you take one with you, let it go, and have it fire at you in reverse. If the mag beam device was low enough in mass to make that work, you might as well use the energy to dump it into propellant and fire it out the back like a regular rocket.

Like space elevators, I could see this as part of an established space transport system, but not much for the exploration phase.