The main argument against voting third party is that it would spoil the election in favor of one candidate or another. However, this argument does not seem to apply to Gary Johnson. The poll data seems to indicate he takes roughly the same amount of votes from both candidates.
If you would have voted for Hillary over Trump but voted for Gary Johnson and if someone else would have voted for Trump over Hillary but voted for Gary Johnson, then the net effect would be neutral for Trump and Hillary and 2 votes for Gary Johnson. That is the basic idea behind The Balanced Rebellion.