That you are concerned about how a libertarian may vote is ample proof that they are not irrelevant.
Did he say irrelevant (I think he clearly understands the need for each and every vote otherwise this whole thread would not be listed), a one vote difference in a close election is possible. I think we feel that the vote would be lost for his party (just like voting for the communist/green/ect party) and would likely decide "JUST" the outcome of the other main canidates. That makes each vote count and relevant. Make your informed decisions on how best to vote, no one can change ones pattern in the voting booth and I am afraid that just bitterness will overcome rational.