who cares when it saves days by the dozens. Even if docker burned all the hard drive in my home right now for some reason, the amount of time gained when I get an user reporting a bug on $OLD_UBUNTU where I can just run `docker run -it ubuntu:14.04` and start reproducing in a couple seconds, that would still be worth it.

Adding complexity you don't need is a bad thing, but the alternative presented is what exactly? Build packages of my software for each distribution my customers use, and tell them to write custom scripts that will bootstrap chroots on each production machine, install the software in there with the distro's package manager, then set up systemd unit files that will run those with systemd-nspawn?

Like with any layer of abstraction, you can't mention the complexity it adds without also mentioning the complexity it removes.

It has encouraged people to move to a more immutable architecture, which makes many types of common problems easier to solve.