1. Gary Johnson is polling well over 20% in some states.
2. Most Republicans reject Trump, and most Democrats reject Clinton. Given Johnson “Tea party” economics combined with the Libertarian policy of toleration on social issues, Johnson has a realistic chance of carrying some states.
3. Given the choices of Trump or Clinton, Johnson/Weld is the unanimous “second choice”. In a state in which Clinton or Trump has no chance, it will encourage their supporters to vote for Johnson in hope of stopping the one they hate.
4. If Johnson does get some electoral votes, and the election goes to the House of Representatives, they vote by state (i.e. New York gets one vote), and states in which the the Representatives are evenly divided will abstain, thus the result could be that Johnson would end up winning even if he came in third (since the hatred of Clinton and Trump precludes a compromise).