There's no legitimate reason for anyone to install a rootkit on someone elses computer.
The only time a rootkit may be applicable is if a companies IT department has some sort of security/encryption software that they want hidden. But at any rate, installing rootkits is a poor policy, because other malignant software could use the installed rootkit to hide itself & remain undiscoverable forever.
Sony was using the rootkit to hide the copy protection software.
Issue 1: The copy protection software was preventing you from using the media you bought & paid for.
Issue 2: Sony installed a rootkit that hid files. This rootkit could be used by malignant software writers to hide viruses, spyware, keyloggers, spambots, zombies & other nasty things.
Of the 2 ... no#2 is the most damaging one, and I'm surprised the DOJ didn't take Sony to the cleaners for employing rootkits without user knowledge, and exposing every customer to potential identity theft & terrorism.
Rootkit a few zombies across the USA & you have yourself an undetectable botnet to carry out DOS attacks practically everywhere in the world.
Good onya Sony!
Here's a new slogan ...
Supporting terrorism since 2006