That perspective is exactly what the demopublicans (Bot's term) want you to have. And, as long as you and the public cling to it, there will never be the kind of change the country needs. Personally, I don't see a vote of one's convictions and beliefs as being wasted, but one for the lesser of two evils is.
Ed explained it perfectly. This is not gambling and the voting booth is not your bookie.
It's about voting your conscience, not picking a winner.
That perspective is only correct when the third party folks fail to provide a candidate that has a chance.
Each time a candidate has come up to "fight" for the Presidency it has been a renegade candidate which captured the public's attention. Problem is the party [usually Independent or Libertarian] fully lack an infrastructure to support such a candidate.
So it is easy to blame the "demopublicans" who just want to "keep down the masses" but the masses have to do their part, and that just hasn't happened. So what we have had, to date is a third party that acts purely as a spoiler, siphoning off votes from one side or the other and providing the minority candidate with the win.
Once the Independents or Libertarians get their shit together on a national basis, THEN the argument that the third party vote is wasted will be nullified.
As for "It's about voting your conscience, not picking a winner." NO, it is about making a choice that is best for the country AT THIS POINT IN TIME. Choosing a Third Party candidate might be a nice point on principle but does nothing to move the country forward if it is fully well know that he/she is going to get < 4% of the vote.