A vote for Romney is a vote for Romney (R).
A vote for Obama is a vote for Obama (D).
A withheld vote is a withheld vote.
A vote for Johnson is a vote for Johnson (L).
A vote for Goode is a vote for Goode (C).
A vote for Stein is a vote for Stein (G).
A vote for Anderson is a vote for Anderson (J).
It is mighty arrogant of you to assume that every withheld vote and every vote for Johnson, Goode, Stein, or Anderson would automatically go to Romney if those four weren't running. Truth is, votes for a third party help EITHER main candidate.