bug#3566: 23.0.94; explain why not to use group defined by define-minor-mode

In the Elisp manual and in the doc string for define-minor-mode, we
are told not to use the custom group that is automatically defined for
the mode variable, unless we have explicitly used defgroup to define
the group.
 
Why? Please provide some explanation.
 
If you use `define-globalized-minor-mode', then the generated group
(with the name of the _local_ minor mode) is recognized by
`customize-group', and the mode variable for the global mode is
recognized by `customize-variable'.
 
Why is it inadvisable to use the same generated group for other
defcustoms without explicitly defining the group? What negative
consequences arise if that is attempted? Please explain in the doc.
 
The Elisp manual, node Defining Minor Modes, seems to refer you to
node `Group Definitions' for the explanation of this warning, but
there is no such explanation there:
 
"*Warning:* don't use this default group name unless you have
written a `defgroup' to define that group properly. *Not
Group Definitions."
 

In GNU Emacs 23.0.94.1 (i386-mingw-nt5.1.2600)
 of 2009-05-24 on SOFT-MJASON
Windowing system distributor `Microsoft Corp.', version 5.1.2600
configured using `configure --with-gcc (3.4)'