For some time we have some apps which chaotic placed on wb there and there , and the only way to have something in one place was always kind of primitive snap-shooting of icons and amidock. It works , and can be used as dock-panel for just apps launching (through there a lot of moments need to improve, and the visual output to user was always pretty primitive there), but when user do not want anything other but just few icons to launch in amidock panel at all , or he do no want panels at all and so on: for those situation screen bar modules/plugins are way to go.
As example of usual screen-bar modules can be: volume control, keymap switchers, clock, amigaamp small-navigation panel and all the usual "tray-kind" stuff, which is not fits to just "let's iconify it to WB" or "let's make it as docky".
All the oses have it of course and we not should go the same way only because "all oses have it", but because it prove to be user friendly, easy to use and just "right" thing and does not matter on what os is.
There is some examples.
WinXp:

Linux/Gnome:

MacOS:

Morphos example 1 from jPV:

Morphos example 2 from jPV:

As you can see, it is present on all oses, and even on our closest to us morphos (which prove to be usable on amiga-kind os as well). Usually there is volume controls, keymap switchers and some other stuff which i point out before. Dbl-clicking on it spawn a settings of those modules and they all holds on top bar just like commodities or so.
For example to have digi clocks on top-bar, AlexC just make a "border less window which stays on top bar", what kind of sucky, because its just one window of top of another and it didn't 'connected' to something at all. So just visual hack if that can be called like this. The same will be with all the others stuff which we want at top bar (keymap swither, volume control), then all will overwrite all together, user will need manually to move it on necessary position, i.e. : primitive, old and non-user friendly problems. With introducing top-bar modules everything will be fine in that terms.
So.. After small intro , the reasons of that thread is to discuss how to do it all from programming point of view. I.e. what and how it all should reacts on amigaos and so on.
I think currently about that (can be very wrong, because know not so many about):
1. screen bar modules can be something like commodities (i.e. the same code, the same stuff). Maybe they even can be commodities at all (so "exchange" usage and co).
2. workbench top-bar module, should have some new hook or so introduced, which will allow to insert/remove plugins from.
3. introduce new flag or so for commodities based code (if it will be commodities), with some necessary options which will says that it is screen bar module
4. they should have inbuild functionality for "remove" and "add" them from users side. So those ones who want will have them (or only those ones they want), those ones who do not want them at all, can remove them all or keep only few as well.
Let's discuss it all, and when everything will be sorted to some sort of "we need x,y,z", i can then make a feature request to os4 BZ.