xfce4-indicator-plugin - how to prevent things appearing there

[Chris Green]

Is there any way to stop something displaying an icon in the indicator
plugin?  I use Thunderbird *only* for its address book but it insists
on putting an 'envelope' in the indicator plugin that I don't want.

As a more general question what does the indicator plugin offer that
applications can't do directly by putting an icon in the panel?  As it
is it seems to just complicate things because some programs put their
icon in the panel directly whereas others go via the indicator plugin.

-- 
Chris Green