I will never purchase another movie. Why? Consider this:
It costs $20 to buy a Blu-ray or $3 to rent it. In other words, you could rent the same movie six times and still save money.
People rarely buy movies before they have seen them, but after you have watched a movie once or twice the joy of seeing it goes away. We trick ourselves into thinking that we will be able to re-experience the feeling we got the first time we watched it. We never do. Movies are not music and they rarely get better after the third viewing.
Look at your Blu-ray or DVD collection. How many of those have you watched seven times after you bought them? I have about 50 movies. The Lord of the Rings is the only one I have watched more than seven times. (But only if you combine views of the VHS, DVD, and Extended Edition DVDs. Which you technically should not.)
What about a sale when you can get a Blu-ray for $10? Well, how many Blu-rays have you watched more than four times after you bought it? In addition, it is going to sit on your shelf forever and never be used. You can find better pieces of art to decorate your house, trust me.
Besides, there are tens of thousands of movies out there waiting to be experienced. Why spend two hours watching one you have already experienced instead of experiencing a new one?