The init scripts for debian and ubuntu rely on a special user:
It has three tasks:
1. Doing some local checks starting the mysqld.
2. Some fancy status reporting.
3. As mysqladmin is used to shutdown, the user is used to shutdown the mysqld.
The problem with this approach is the credentials has to be stored disjunct in the filesystem (/etc/init.d/debian.cnf). So everytime the data in the schema changes you got to fix it in the configfile.
So at least when you i.e.
Create a slave from a master dump
After an (initial) SST in Galera
When you miss this, the most important thing is, you are not going to stop the mysqld.
This is some kind of silly, as you don't even need mysqladmin to stop mysqld, as SIGTERM is sufficient.