Right now it is possible for add-ons to add their own menus and menu bars,
however that is sometimes not the right thing to do. Sometimes you want to offer
functionality in an add-on without forcing a certain UI on the user. This is
especially important when your add-on offers functionality that needs to be
available on every desktop but is not supposed to be visible by default (e.g. to
prevent untrained users from messing things up). 

This is currently an issue for our organization because we're developing
functions to help users create form templates. These functions need to be
available on every installation, because the creation of form templates is not
restricted (every user who knows how to do it may do it). However, as these
functions pop up dialogs that will confuse untrained users and may cause them to
break existing templates, the functions should not be visible by default.
Furthermore, those developer functions are too few and not used commonly enough
to justify adding new menu or even a new toolbar.

The proper way for an add-on to offer functions like this would be to add a
custom category in Tools/Customize../{Menus,Toolbars}/Add.. that contains the
different functions that the add-on can perform. The user can then decide
himself which functions he wants to access and into which menus or toolbars he
wishes to place them.

This RFE requests that a way be added for add-ons to offer

a) their own categories in Tools/Customize../{Menus,Toolbars}/Add..

b) their own functions in the custom categories

c) icons for the respective functions

A workaround until this feature is implemented is to add a macro library that
provides access to the functions in question. Unfortunately this has several
drawbacks, e.g. that the library is exposed to the user and that no default
icons can be provided.