I look at in a couple of lights:
1) Lesser of four evils (even though he won't get elected). This, then, becomes an incoherent argument for support. But his positions are more amicable than those of the other three. Then who is realistically the next in line? Gore? His policies aren't too fantastic either.
2) Protest vote - I see the logic, but it should be framed in the context of a support vote. That is, provide support for the Greens come future election cycles. This, I think, is a worthwhile cause.
3) Abstaining - I don't think the system is capable of fixing itself. I highly doubt that politicians will say "Uh oh, people aren't voting. Let's reform the entire system." I would venture a guess that most elite power-holders would prefer to see people not vote. That just means the less they have to worry about renegades and other annoying people.
Aaron