Yeah, I wouldn't expect to see all the nasties without a long-term histogram. And a lot of it is protective "just in case" stuff that happens frequently enough to be a real concern, but not necessarily every day. I'm sure you'll find leagues of people that say "hey, I ran mine from a 7805 for three months, and it's doing just fine!" Good for them. It might run forever. But it might not, and it's not too hard to protect it properly, so why not, eh? Better than having it spontaneously croak some random day for no apparent reason.
I just started a thread here in this forum for a design review of a suitable automotive PSU. The one you posted looks like a great start, and is really close to what I came up with. I would still avoid the linear regulator if you could. It's just bad practice to turn excess voltage into waste heat when you're running from a battery. That's your call though. If you drive it every day, you'll probably keep the battery charged enough that it'll never matter.
I still maintain you're making things complicated for yourself by cutting power to your Arduino, but it's your design and your right. 