I think some people do, FlyingConsultant! It's not good though as you should be awake during descent to clear your ears. Really.
I think the 20 minutes or so before landing is because the cabin crew have to tour the cabin to make sure everyone has complied (amongst doing other things) and not leave anything until the last minute. I'm not cabin crew so I am hazarding a guess. There are always some that have their tray tables and seats reclined up to the last minute (and trying to go to the loo on short finals) which I believe really is a safety issue.