Voting what you think will happen is the only way, otherwise it might as well be in the fantasy tennis section. I don't vote for Roger against Rafa in slams since 2008 'cos frankly it'd be downright ridiculous at this stage. Can't see why people just choose the player they support even if deep down they know they haven't a prayer. It's not a poll then, it's a comedy sketch.
Also, Novak v Roger is a much more even contest as Roger v Rafa is a one way contest at this stage so voting for Roger against Rafa is just plain dense. I mean would you bet 5k on Roger to win just 'cos he's your favourite player? I think not..and good luck with that if you do!
And that should be the governing thought process for polls. If you had to bet money, who do you REALLY think is going to win. Otherwise what's the point of the poll to begin with?