We went through a driver training for work that told us to do this. Personally, I think it's a false sense of security. If you get used to not checking your blind spots and get into a rental car, don't adjust the mirrors, etc, you're setting yourself up. Not to mention it eliminates something like 90-something percent of your blind spot...not all of it. Which means that if you have a motorcycle riding there, you need to check. I adjusted my mirrors to the "correct" way, but I still always check. I'd rather be paranoid and safe than oblivious and sorry. Not to mention, with how much I drive for work, I move around a lot in the car. I slouch, I sit up straight, etc. Which affects the way I look in my mirrors, especially my rear-view. I can't imagine anyone who has the same posture the entire time they're in the car.
Sorry, I was feisty during that entire training session. I don't believe in giving people a false sense of security. Honestly...you can't get in the car with everyone and tell them that their mirrors are adjusted properly. I'd rather have someone check their blind spot and not rely on their mirrors.