I agree with You that voting for the lessor of two evils when the RESULT would be the same is pointless. For example, a Romney or Obama vote is pointless.
Hypothetically, if RP were pro choice, would you abstain from voting for him too in a contest with McCain? By your definition, RP would be the lesser of two evils, and therefore you would not. If that one vote were the deciding factor, would the world be a better place, worse place or stay the same?