If you were a malicious user with root access to a machine. What would you do?
Install a rootkit? start a botnet farm?
I can't help but think that I won't just do something as simple and annoying like
# echo "echo b > /proc/sysrq-trigger" > /etc/profile
If you have a modern distro that doesn't use bash as part of the boot process, then it could be a very long time before before you notice something is wrong.
You can't debug it, login to single user mode etc.
Repeated forced reboots might even cause some hardware damage.