I don't believe in the notion that there is such a thing as throwing away your vote. However, not voting for one of the top two is the same as saying that you are ok with either of them winning and I am NOT ok with Obama winning. I am not ok with the policies that he will (not may) try to implement, and I am not ok with the judges that he will (not may) nominate.
Since this is a gun forum, I'll stick to the gun stuff. I've heard a lot of people in a lot of places make comments amounting to Romney is anti gun, yet I have failed to see anything that truly backs that up. The GOP has adopted one of the most pro gun platforms ever, but now many gun rights supporters are no longer backing the GOP. We get what we want, but now it's no longer good enough? We are worried about flip-flopping while we flip-flop ourselves.
I didn't vote for Romney in the primaries, but come this November I will happily be voting for him then. To not is to accept Obama, and that is exactly what he wants.