Nah, it's really more of a utility-based argument.
I'm going to just make up some numbers and a hypothetical voter here, for the sake of argument.
Ok, our hypothetical voter prefers Jill Stein to any other candidate. The Democrats do a lot of things that he objects to. Of course, since the Republicans do even more things that he objects to, he'd prefer a Dem than a Republican in the White House.
Let's give his options some marginal utilities, then. His vote will only make a marginal difference in the outcome if he votes D or R, so we'll assign those votes +1 and -1 utility, respectively. But wait! There's some value in the third party you support receiving votes, even if it's not as much value as winning an election. Let's assign that marginal value +.1 for Green and -.1 for Libertarian.
This is why people get angry. They look at this voter's values and say, "hey dumbass, vote Dem! The marginal utility is higher!"
But that's not the end of the equation. For this hypothetical voter, voting for somebody who does things he doesn't like feels like supporting those things. It causes him a certain amount of mental anguish. And, on the other hand, voting for somebody whose platform he fully supports feels like making a stand for his beliefs.
Add those to the equation, and suddenly the utility of voting Green is higher than the utility of voting Dem, for this particular voter.
And you know what, I completely understand that. If for you, supporting someone who you actually believe in outweighs the marginal value of your vote, I'm not going to tell you you're some kind of horrific monster. The way you feel is the way you feel.
But on the other hand, I can also understand the folks asking you to try to put aside those feelings and put more weight on the marginal value of your vote.
