Relative vote totals count. A legislator elected 80-20% will act in a certain way, a legislator elected 50.1-49.9% will act in quite a different way. Today, you can see this written into news stories about Obamacare affecting "vulnerable Democrats", but it's always something; the 50.1%-er is always looking behind their back.

Mind you, this still doesn't mean that "your vote counts" in a huge way or anything, but it is false that the only vote that counts is the one that swings the election. Votes do not merely choose whether Person A or Person B "wins"... it also imbues the winner with more or less power and flexibility.