What ever happened to being able to own the product you purchase?
If I have to log in to someone's membership site to view the material everytime then do I really own that product? What happens when they decide to take the product down and I want to go back to that same product to look over something again?
Please, allow your customers to download the videos they have paid for. It's kind of like walking into McDonald's to buy a burger and the lady at the counter holding the burger box whilst you eat it. Shouldn't you be able to walk out the door with that burger you have paid for?
If you are worried about people sharing your videos, well I have news for you. People who want to share your videos will do it regardless of whether or not you have them downloadable. There are plenty of programs people can use to rip them straight off your site. In fact a lot of times the harder you make it for them the more interested they are in trying to crack through your security.
Besides, you should be catering your site and content to your paying customers NOT those small minority who might redistribute your videos.