Not voting is the same as voting for both, or saying "neither".Not to argue, but not voting is not apathy. Not caring is apathy.
Not voting says, "Both of these choices suck, I'm not going to vote for someone I don't believe in, just to spite someone I believe in less".
If you really want to undermine the Republicans, vote Constitution or Libertarian. They both steal votes from the Republican party faithful directly. Supporting one of them (Barr, Baldwin) would go a lot further towards screwing the GOP than voting Obama.
But that said, I don't think everyone should vote. People that are uninformed on the issues, or don't have a real interest? Why the hell should they pollute the system.