My personal opinion is that you should never vote in a system that is rigged/unprovable on the principle that your "vote" may or may not actually be counted but your participation is certainly counted on to give the result legitimacy.
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I respect this position, but I think it's flawed. The issue is the implication that higher participation can somehow give the result legitimacy or moral weight.
I would argue that it can't, and thus that voting is an amoral act of self expression. Certainly a very limited form of self expression, and one that's frequently taken out of context by those claiming it grants them extra moral rights, but the fact that others misquote you and take your words out of context is not an evil act on your part.
A vote is an expression of an opinion and is amoral.
An election is the aggregation of everyones opinion which is then used to justify some action, usually government coercion of some kind.
The legitimacy of the aggregation of everyones opinion depends upon the number of people who contributed their opinion to the aggregate.
The corruption of societies ability to assess the aggregate opinion results in abusive immoral power derived from fraud. Fraud is immoral.
Participating in a system designed to facilitate corruption means you are not actually casting a vote, you are supporting a fraud. Your opinion cannot be provably expressed, but as byte master said, your participation can.
An opinion that supports coercion when publicly expressed by voting ends up enabling coercion and is not a passive act. If no one voted then no one could claim they had public consensus on the use of coercion. If you have ability to file an active protest vote against coercion then that would be the most moral thing to do. In this case you can simultaneously reject the system and help project public consensus that voting is not a means by which coercion can be justified.
So I agree with Bytemaster that voting in any system that allows your voice to be stolen and reprojected is irresponsible toward your fellow citizen and gives your power away to someone without your consent. Expressing your opinion in a system such as VOTE will allow you to be responsible for standing up for what is right.
So it really comes down to an assumption on the part of those who say you should vote in the current system. That assumption is that the current system actually works as intended. If the entire premise of your product is that the current system does not work as intended then the only logically consistent conclusion is to be pro-active in your stance that the current system should be shunned entirely and that is the RESPONSIBLE thing for a voter to do. It is a VOTE OF NO CONFIDENCE.
The purpose of voting is to make your voice and opinion known to all. Bad things happen when good people don't speak out and silently consent to the evil. So it is imperative for your own safety and wellbeing to publicly express your opinion in an honest voting system such as VOTE. So the argument for VOTE is that you need to let your voice be heard and stop letting your voice be stolen and manipulated.
So everyone that says you have a moral obligation to vote may be right in principle, but that principle only applies when voting has integrity. What they are really saying is that you have a moral obligation to contribute to the aggregation of public opinion. This is what VOTE gives you.