The point is that your vote adds a (very small) incremental value; the sole value is not that it might decide the election.
In regards to this argument, a single vote has no value because the outcome of a single mandate election of this scale is never determined by 1 single vote. The outcome would remain the same with or without your vote. That is the point.
You bring up issues and demographics. Same rule applies. The attention given to your demographic is never affected by 1 single vote unless it is the single vote that causes the politician to focus attention or turn his/her back. This is obviously a gray area that can't be measured.