Oddly enough, there's a very similar discussion going on in leftist circles in Israel, despite the fact that we basically have a multi-party system. People who are left of center end up voting closer to the center than they'd like just to make sure their vote gets counted. But in Israel as in the US, not voting doesn't empower anybody. It just allows your perspective to continue being ignored. Of course the system is geared to encourage this. In the US more than Israel, I'd say. Voting doesn't make anything better, but not voting contributes to things going the way they're going already. I think the only solution is to vote for a party that has more than a snowball's chance in hell and focussing your energy on changing things outside the electoral system. Voting is just putting your finger in the dam. You still have to work on repairing that hole or pushing back the tide, and that requires real work, not just putting a piece of paper in a box.
BTW, in Israel there's a significant movement of lefties registering in Likkud (the big right-wing party) in order to participate in primaries and try to keep them from veering off too far to the right. They then vote for a leftist party come real elections. It's a response to right-wing extremists (settlers) who do the same thing, except come elections they vote for the outright fascists, whom their plants in Likkud give a place in government. In America you have the Tea Party doing that kind of thing to the GOP, and I wonder why no lefties have registered as Republicans to sabotage that effort (or maybe they have and I haven't heard of them). Probably because of how entrenched partisan lines are and because of the idealistic hope that if the GOP goes off the far end they'll never win an election again. And I think we're starting to see that people are way too stupid to be expected to act that way. The GOP has for a long time been really bad for most of its voters and outright crazy, but they manage to win some races anyway because their tactics aren't based on sound policy. Even their politics aren't based on policy.