A large number of malware sites will walk you through side loading and bypassing your security to install their app which may be a compromised game or app like a Trojan. Not having this access is the only way to deal with this.
Some minimal level of responsibility of action from a users side isn't unreasonable. Being locked out from your own hardware because there are a few uneducated users out there is absurd.
Not to mention that now there are actually viruses and exploits out there that install themselves through security holes on users devices and the users can't remove them because they're locked out!
If you got an older iPhone, it's best not to open random PDFs anymore.
I was kind of thinking more of examples of malware where people were actually using the lockdown of the OS to stop users removing it, but hey I said exploits and that's an exploit.
I think CyanogenMods solution, where root access has to be enabled in the Developer Settings, which have to be enabled by tapping the Android version number 6 times, is the perfect solution: My mum would stop immediately doing such a complicated thing if she wanted to install something.
That seems to me to illustrate the point of a locked down environment fairly well...