So why do the developers give the backup advise very clearly and definite when a new version comes out?
Hey, I give that advice myself. And why we give it is real simple: Because you never know. Like I said, I have automatic daily backups. And I post on some of those sites about once a month at best. :) Space is cheap, backups are critical.
I'm not saying that backups are not important, I'm just saying that manual upgrades are frankly quite simple, really.
I will state it again: WPAU works (worked, sadly) out of the box, the native WP2.7-update does not, at least on some installations. That is a fact and nothing you write here will change that.
You can't change it because it's not true. WPAU fails critically on dozens of people's systems every time an upgrade comes out. Just look at these forums, you see loads of people with install problems, and many of them used WPAU. It has a high failure rate, and we repeatedly have to tell people to install manually.
Saying "it just works" when it does NOT just work in a large number of cases really doesn't help your argument. WPAU fails just as much as the built in updater does, and usually on the same hosts.