Your same argument that it's pointless to vote for either of the major parties also applies to voting for a third party. Your vote doesn't matter either way. You could easily say "who cares if Nader loses by 2,304,418 or 2,304,417 votes?" Regardless, I think people don't vote for a third party because they want to prevent a greater evil by voting for a lesser evil. Someone will vote for Gore if it will prevent Bush from becoming president or vice-versa. If someone either does not care which candidate wins or if one of the candidates has a definitive lead, then by all means vote for the third party that best represents your views.