It is worth voting to me only if three conditions are present: (1) I have some kind of interest in the election, (2) There is a candidate whom I want to win, and (3) I can trust that the outcome of the election will be a result of the votes that were cast.
With that said, I don't vote. I don't actually believe in representative government, especially the way it works in the United States, but I would participate if the system seemed legitimate and there was a worthy candidate. Unfortunately, politicians are largely scum, the government is corrupted from top to bottom, and much of the electoral system is run by a politically-aligned private corporation. Voting is basically renewing the lien on your tax money and re-surrendering your political power to corporate America every couple years.
Not voting in and of itself is not an effective way to protest government. There is no statement to be made there, as non-voting for any reason, be it protest or laziness, all gets lumped together as one unheard voice.
Is the system corrupt? Yes. Is the government corrupt? Doubly yes. But not voting is not going to change that. Not every candidate for every post is horrible, incompetent, or tyrannical, but the genuine do-gooders rarely make it to positions of decent influence. People who sit back and don't vote are usually either too apathetic or too jaded to seek these people out and support them.
But how do we get more people to vote? The trouble seems to be getting people to the polls...because theories such as the one discussed in this thread are prevalent. People think that by not voting they are involved in some sort of protest, when in reality they are hurting the cause they want to support.
I don't believe in voting, because I believe that the system is inherently corrupt, and that voting only endorses its continual existence. Of course, not voting (or spoiling my vote, which is what I will do next election because it's more symbolic) had to be matched by a commitment to other forms of action. Not voting isn't really a form of protest, unless you get lots and lots of people to do it. But it is an important principle, for me at least.
If you want to support your local do-gooders, why not get together and form an organisation they can do some good in? Look at the example of the nationalist communities in Northern Ireland. Isolated and totally neglected by the sectarian Unionist government which gerrymandered them out of political representation, they created their own community leaderships which turned sinkhole estates into proper communities.
I think that by spoiling your vote you are essentially telling the majority that they can go right ahead and dictate how your life is run. Although I understand where you are coming from with your oppinion, I think it is just another way you are going to be screwed by the masses and the politicians.