Comment 9 for bug 23369

I think the idea is to reuse existing interfaces that do what you want instead
of creting new custom ones... This _definitely_ applies to the observer
interface. nsIObserver exists so we don't have hundreds of differently named
observer interfaces that all basically have the same method on them.

As for locking; if I understand correctly, the code, as written, will mean a
user cannot set a pref in Mozilla to override the system pref. That seems
backwards to me.