Is there a scenario where rm -rf -no-preserve-root is needed?

IMPORTANT: Modern UEFI systems mount the firmware under the /sys directory and make it available to the OS. DO NOT run this command on a modern system since it will remove this firmware, essentially bricking your machine.

The simplest scenario I can think of is someone wanting to delete all the data from their drive. There can be perfectly legitimate reasons to do this and the simplest way I can think of is

rm -rf --no-preserve-root /

Turns out this one is actually given as an example in info rm:

`--no-preserve-root'
    Do not treat `/' specially when removing recursively.  This option
    is not recommended unless you really want to remove all the files
    on your computer. 

Another perfectly good reason is that you want to delete a mounted file system that you've chroot-ed into. In that case, rm -rf --no-preserve-root / will delete the system in the chroot environment but will leave yours intact.

I am sure there are more possible reasons, but in general it seems a very reasonable approach that my system allows me to do whatever I want with it. It's my job to be careful, the system should only enable me to do what it is that I want done. If what I want is stupid, that's my problem and not the OS's.

Anyway, this is a relatively new restriction, it was added in the 7th version of the POSIX specification (the previous one is here), before that rm -rf / was a perfectly valid command. On a historical note, the . and .. directories have always been protected from rm, ever since 1979, when rm first acquired the ability to delete directories. More on that here.

The existence of the --no-preserve-root switch is not to add additional functionality but to override a very sane reduction in functionality. This switch is likely based on the philosophy that the computer should do what it's told and that commands should be available to express any desired action. This switch pre-dates UEFI, and based on my experience, I say it's now obsolete.

In modern practice, without this switch, the rm command avoids the accidental deletion of the root directory when using an uninitialized variable or a stray space.

rm -rf /${my_directory}
rm -rf / var/log/httpd/*

Fun footnote: Protection was not its intention. Per a Sun Microsystems blog, removing the / directory will implicitly remove the current working directory, a violation of the special consideration already made for the . and .. directories. This is why their standards committee allowed this special exception--not for preventing an accident. This change was first introduced with Solaris 10 build 36.

http://archive.is/5lmc9

Perhaps this is not an answer that original asker wants, but there is a use case that needs to delete all files recursively from the root directory. Although it's not through the rm command, it's part of switch_root(8) process during the booting of your (non-embedded) Linux system.