Humans perceive the world through their senses. The degree to which we rely on a particular sense varies depending on the situation. Visual cues often can occupy our cognitive load, diminishing the amount of cycles given to other stimuli. Part of that is lizard-brain stuff - we are tuned to visual motion because it could be a threat (e.g. something coming to eat us). So if you're concentrating on what you see, you have less of your brain to focus on what you hear and feel.
This is why I prefer unlined. I use the side dots as reference, but my mind knows that it is just an approximation. So the amount of brain power I'm using to "see" where I'm supposed to be playing is lower, leaving more for my ears to process (though some might argue that interpolation might add to the load). If there are lines, my mind thinks that is "the answer", so more brain power goes to the visual cues matching physical movements, less to the aural correlation.
That's my theory anyway, and I find that I have an easier time playing unlined than I do lined. Your theory may be different, and your experience may be different. And that's...ok