A vote for a Libertarian takes a vote away from the Republican just as a voter for a Green (or other fringer) takes a vote away from a Democrat.
It teaches neither party nothing as there has never been enough nation-wider votes to mean a thing and only rarely has it meant much at a more local level, at which point, you get a Gov. Ventura.
Romney is no where near as bad as Obama, but if you want to see Obama re-elected, throw your vote away on a pipe dream like Johnson.
I'd recommend against it.