Comment 124 for bug 390508

So the argument about users and their experience, or how you think it should be used goes out the window if developers can and are starting to get around it. In which case you may as well just open it up properly for use. It's confusingly documented that way, and people have requested the change (with decent use-cases).

If you want to set a global timeout across the whole OS, then it's no good having work arounds as mentioned above.

Take a look at other notifiers like snarl, ruby-libnotify, jquery notifications, or even when writing Windows apps it's possible to set a timeout.
Of course they all must be wrong to offer that sort of override.