Voting legitimizes whatever atrocities your government commits
Symbolism is important: withdraw consent
Voting is the same as not voting, but with more time wasted
Dispiriting to other people, who have to tolerate the charade
Not materially adding to any percentages
Legitimizing the system by submitting to it's theatrical demands
Thinking independently etc.
Recommended:http://www.amazon.com/Democracy-The-God-That-Failed-Economic...
The full intro. chapter is available free: http://mises.org/hoppeintro.asp