mouse clicks on tool-bar -- why not recognize different mouse buttons?

[Drew Adams]

It looks like all mouse click events on tool-bar items have the same effect.
Or, rather, it looks like keyboard modifiers are taken into account, but all
mouse buttons are treated alike.

Except for the keyboard modifiers, this is like the treatment of menus in
the menu-bar (which doesn't recognize keyboard modifiers). This is standard
for menus -- you generally want a menu item to mean the same thing (what the
item says), regardless of keyboard modifiers or which mouse button was used.

However, since a tool-bar item is just a graphic (not a specific name, like
a menu item), why not treat tool-bar items more like we treat portions of
the mode-line: why not recognize different mouse button events on tool-bar
items? Why not, say, be able to have mouse-3 on a given tool-bar button do
something different from mouse-1 on the same item?