That wouldn't be a bad idea if the goal was to somehow make the notifications actionable. That's not the goal. This would still compel the user to react. These notifications are not meant like that. They're meant to keep you updated about what's going on and nothing else. You can read more about the reasoning here: https://wiki.ubuntu.com/NotifyOSD
There is a fouth way to notify users, by the way. That's the window urgency. When an application calls for attention, it'll come out of the launcher, shake and its "window open" arrow will turn blue.