I do understand that that's the point of AP, but don't you think that you will become less alert when you've driven thousands and thousands of miles without having to interfere a single time? Yes, AP frees the driver from all the repetitive tasks so the driver can look ahead and have a better overview of the whole situation, but it's hard to keep that attention after you've been in the car for hundreds of hours and not having to interere once!
And no, that still doesn't mean you should go watch a movie in the car, but even answering a text or looking up what the weather will be like when you arrive or looking at someone while having a conversation with him/her can cause the same fatal result. The problem is that when you have a 'normal car' you have to look out the window and be attentive, because otherwise you will drive off the highway in seconds, but AP is so good that 99,999% of the time you can look somewhere else and still be safe. This doesn't mean that I'm not a fan of AP (I'm a huge fan!), AP has proven already to make your car safer, but this is certainly an issue that Tesla and other automakers need to address in the near future when they have systems that can drive almost(!) autonomously but still require the attention of the driver.