When people say, "my vote wouldn't have mattered anyway", they're not talking about their ability to sway the election results, they're referring to the fact that they don't care who is in power, because they don't see a difference regardless.
Bingo.
While voting is not an obligation it really should. If you don't like any of the candidates and what they offer you can go down to your designated voting station and inform them that you refuse to vote for any of the candidates. This is counted separately and at least keeps a track record of those who are fed up with all parties and want a change in the system.
In other words, it wouldn't make a difference. And forcing the public to vote by making it mandatory is a terrible idea.