I would have to add that I see one element which is never addressed when decrying a vote for [third-party candidate] as a vote for the other side: When you do the math, presuming that you support the R candidate, a vote for the L candidate, for example, has only half the negative impact to your cause as a vote for the D candidate given that it (theoretically) reduces the R vote by one, while voting D both reduces the R vote by one and increased the D vote by one, generating a 2 vote shift in the point spread...
Your math is good. However the math doesn't take everything into consideration.
Votes supporting these "principled" third partiers that cannot contend or be "principled" enough to pull out in the 11th hour for the greater good just feeds their narrative for future runs, hence we are talking about them as viable (wasted) votes again.
We all know some people will fall for anything, it's normally 2-5% of voters. While often not enough to obviously change election results, these 2-5% are frequently the most loud and obnoxious. What cannot be counted is the voters that are disillusioned into not voting at all by the BS and rhetoric created by this sort.
This is the reason we have Joe Donnelly as a senator instead of Richard Mourdock, who could arguably have changed our do nothing Senate. It could well be the reason why white Republicans didn't show up to vote for Romney.
Too often we don't have the capacity to learn that our well intentioned votes of the past have cost us big time.