Let's say you go to see Titanic. You get to the theatre and surprise, sold out. What do you do? Go home? Nope. You go see Gigli, even though that movie would never have brought you to the theatre in the first place. And, you pay the same price as you would have paid to go see the movie you did want to see.
You're suggesting that these people pay less. You're also suggesting that if Gigli was $3 instead of $12, you would actually get off your butt and come to the theatre to see that steaming pile of excrement - knowing full well that everyone reviewing it is telling you it's crap. I think not.
Why do you think the studios position the movies that they know will be very good/in high demand to come out at staggered times? Yup, to ensure that there is always overflow.