Actually, the only wasted vote for undecided voters in non-swing states, would to be to vote for either Clinton or Trump and do absolutely nothing to the outcome. Voting for a 3rd party to help uplift it to get federal funding and more national recognition/influence, if that is something you believe is needed for our future elections, is substantially less wasteful than adding votes to a foregone conclusion.
Yes, your argument holds water in swing states, but to those who are trying to uplift a 3rd party by getting them to 5% nationally (federal funding) and showing the establishment that the message of anti-war, anti-fracking, anti-TPP, combating global warming with vast scale renewable energy production, getting money/corporate influence out of politics, regulating and taxing wall street, implementing universal healthcare, and tuition free higher education is not only extremely important, but also sensible plans for our society, environment, and future generations.
Sadly, it seems, the Democratic party is much too influenced and controlled by centrist/corporatist neoliberal politicians to adopt those positions. Bernie Sanders is trying his hardest, I know, but to begin to empower a backup plan in a situation where your vote won’t make a difference to the overall outcome (deep blue, deep red states), isn’t selfish wasteful voting, it’s actually quite strategic and tactful in the long game of politics.