Blackwaterdog's diary used some strong language, and received some strong responses. I'm going to take a crack at the general issue, but with what I consider a milder argument.
The argument goes like this: in the American voting system, you should support the candidate more likely to act on your behalf, even if you don't think he's doing anything about it right now.
See below for reasons why.
First premise: regardless of the motives of any individual voter in either voting or abstaining, the overwhelmingly likely outcome of every election is either a Democrat or a Republican in office.
You can say absolutely anything you like about owning your vote, or not supporting a candidate who doesn't support you, or anything else. Regardless of any reason you may have for not voting, however, one of those two outcomes will almost certainly come about.
Second premise: The candidate which wins a given election will, more than likely, work to advance the causes of his respective political party.
This isn't always true, or even mostly true, but the Republican voting bloc in Congress has traditionally been strong enough that only now are we seeing front page articles noting its weakening.
Third premise: the other guy is doing everything he can to promote this same sort of thinking among voters. Narrowing the intensity gap is good news, but the gap still exists.
Fourth premise: your voting decision will have consequences lasting from two to six years. If a politician has no interest at all in supporting your cause, you're not going to see any support from that office for a long, long time.
Final premise: the election is not the only time you're able to influence a political outcome. Letter-writing, protests, demonstrations, phone banks, and well-articulated arguments have all swayed elected officials in the past, and they will continue to do so in the future. But they will sway those people to a greater or lesser degree, depending on that individual's political affiliation and present circumstances.
Without being able to predict the future, do you think it's more likely that a Democrat or a Republican will support your goals? Decide who that will be, and then vote for them. If you see a third party candidate you'd rather support, vote for them (but that's another diary). If you are an American citizen, you're already participating in this process, whether you like the candidates you have or not.
I have seen assorted arguments made against this position in the past, and I'll attempt to answer them.
"You're telling us to shut up and vote!"
No, I'm just telling you to vote. Keep agitating, keep posting, keep expressing your displeasure - but vote anyway. Vote, get the guy into office, and then don't give up until he's heard your case.
"I own my vote."
I agree. Now be responsible in how you use it. Not voting in an election means you're giving up your ability to influence the outcome of that election. If you do that, be prepared to accept the consequences, no matter who gets elected.
"I'm making a statement."
Non-votes don't contribute to electoral outcomes at all, except to the statistic indicating what percentage of the population voted. That statistic makes no political statement beyond "I didn't care enough to contribute".