Rational self interest be damned.
Well, yes, voting for emotional reasons is equally valid. The outcome of emotional decisions may not be rational, but they're not invalid because of it.
Some say it's your patriotic duty to vote on behalf of your country even at the cost of your personal welfare.
Even if one votes against their personal welfare, isn't that an outcome they desire? Put another way: if I am a wealthy individual, and I vote for a way that will result in my wealth going away, then that is the outcome I want. An outcome that does not result in my wealth going away is then what I did not want.
This isn't about whether the outcome of a vote is positive or negative for a given voter. It is about understanding the consequences of the winner-take-all system that we have, and how it hamstrings third-party votes. As I stated in my original comment, third parties have and do win sometimes, but such events are rare.
There is nothing wrong at all with voting for a 3rd party. If enough people agree with that same 3rd party, they'll win, and that'll shake up the game. So it may also be considered a strategic choice. The Spoiler Effect is where a given majority of voters are concerned that their choice will not get enough support, and thus they will vote for the candidate they can stand a little more, guaranteeing that their ideal choice will not get enough support.
It reminds me a bit of playing the lottery: the given statement is 'if you buy a ticket you won't win; if you don't buy one you can't win.' Though some people do win. Similarly, 3rd parties can never win unless people do buck the Spoiler Effect and vote for them anyway.
So: Your utilitarianism is an assumption not a conclusion, and the outcome you've chosen to optimize for is also arbitrary.
The act of voting is in declaring one's preference, is it not? The outcome desired is one's voted preference. That doesn't seem arbitrary. Though this whole Brexit thing, where people voted for an outcome they apparently didn't want, is still sort of boggling.