Re: Binding a command to the down-event of a toolbar button

[Richard Stallman]

    If they were in the buffer itself, then they wouldn't depend on which
    buffer was selected -- they would always be displayed, so the user could
    always click on them, even while working in another buffer.  The tool
    bar changes according to which buffer is selected, just like the menu
    bar changes, because they are defined as keymaps.

    If the buffer happened to display a long playlist, the header line would
    be a good place to put the control buttons.

That seems like a good argument.

And Emacs already supports the desired feature, for icons in the
buffer or in the header line.