As much as I can see that people might like this, I, _personaly_, don't like it at all.
The traybar is a place were icons are put for important programs which are needed to run constantly (in the background) and are kind-of needed for the system. It is not the place to put all the flashing icons from incoming messages and stuff, because that is why that taskbar is there. In other words, don't turn the basic Windows functionality it around.
I get the jiggles when I see a traybar like this:
including that left arrow, meaning there are tons of more things which even don't fit all there....
But, as said, this is a personal opinion. Just to say that there also a lot of people which don't like this kind of stuff. But for those who do like it, it would be a nice feature to enable (and already requested before).
Btw, a good alternative is to use grouped conversations using plugins like you could find here:
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CookieRevised's reply to "A window for all" plugin?"