Perhaps loud suggestions like these are necessary due to a bias in the group that uses Docker. 20% people that really know what they're doing and have chosen Docker for a specific reason, and the hangers-on who try to emulate them by using the same tools.
Docker is an interesting, useful, and extremely overhyped tool. I may be wrong, but I feel like its popularity has caused a bunch of people who don't really need Docker to use it. Besides popularity, they can't really explain why they are using it over something like LXC or FreeBSD jails.
I imagine (again, no data to back this up) that this same large percentage of people are also the ones that just keep using the default Ubuntu image once they finish the "Get Started" tutorial.
I'm glad to see suggestions like this gaining popularity. If you're going to make the most of Docker, I think there's value to be found in really committing to the mindset of a 'purpose-built, no-frills environment for running a single process.'
From what I can tell right now, a huge number of people are using Docker "sort of like a VM but you need more of them, and Git is integrated and you have to tell it to do something or it stops running".