For some reason, they changed their mind when their idealistic stupidity meant Bush won. I think you will agree that their votes did make a real difference, especially to Iraq.
Voting IS idealistic. You're more likely to die in a car accident on the way to the polls than you are to change the final result. You spend bus fare, wait in line, present ID, educate yourself about candidates, etc. If you're completely rational, then voting is a bad idea; your average utility will decrease. Even if your values include the welfare of others, there are much more efficient means to achieve it in the same amount of time.
Most people unknowingly vote due to superrationality - they think about what if everyone like them skipped voting. Then the Bad Guys win! If you're willing to go to all that trouble to make an insignificant difference, why then take a step back and vote tactically, and with a one-move-ahead strategy like that? If you're using "if everyone did that" reasoning, shouldn't you vote for whoever you think everyone else should vote for?
It's popular to hate on Nader supporters, but far more people never voted at all, or voted for Bush. Nader supporters didn't ask for a broken plurality voting system - they're just answering honestly. Maybe you should blame a Democratic party that has no intention of fixing the flaw that led to Bush because it keeps them in power too.