I do use an Android phone on a regular basis, and there it's useful, since it only pops up when important messages arrive (read: on-call stuff).
But my daily driver is an iPhone, and sorry - this would be an absolute mess with the amount of apps installed trying to notify me of things. It's virtually impossible to determine what notifications or messages are important. Is a waze notification that a friend will almost arrive important? Not really. Is a waze notification that I should leave in 10 minutes important since the traffic isn't optimal? Absolutely. Relying on such an indicator simply doesn't work the moment you have dozens and dozens of apps installed that all send you mostly mundane low priority messages, but from time to time want to tell you something you really want to know.
In my case, a light like that would be on all the time or off when you actually have a pretty important notification - which would mean you simply cannot rely on it. Notifications are already too complex, and at the same time too limited. Simplifying it into a colored light is just adding to that mess.