If you refuse to support McCain you'll have to bare a measure of responsibility for Obama or Clinton winning. I would take this argument a good deal more seriously, I suspect, from someone who actually knew the simple difference between "bear" and "bare."
More to the point, however: horse apples. ;) Even baseline, grammar school-level logic should serve to (re-)(re-)(re-)(RE-)illustrate the inherent silliness of said assertion. To wit: if (by your logic) any ballot not cast on behalf of Juan McCain is, de facto, one to the benefit of Cankles and/or The Obamessiah... then what, pray tell, is a ballot not cast on behalf of either Obama or Hillary?
Logic 101: if "A" equals "B," then "B" (likewise) must equal "A"... or else your theory is, quite simply... welllllllll...
any ballot not cast on behalf of Juan McCain is, de facto, one to the benefit of Cankles and/or The Obamessiah... then what, pray tell, is a ballot not cast on behalf of either Obama or Hillary? That's a lot of verbosity to miss the simple plain fact that it depends who the voter is. It's a pretty safe assumption that a Freeper vote is a potential Republican vote. If someone who otherwise would have voted Republican stays home sulking or writes in Mickey Mouse, that is a GOP vote that's been lost. That is how a vote not cast for McCain BY A GOPer is helpful to the DEM party. Every two sulkers equals one vote for Hillary/Obama.