nesting colgroup and rowgroups

The colgroup example discusses grouping permanent and local contact
information separately.  Even within these groupings, there may be
subgroupings, such as physical address vs phone number.

I would prefer that a colgroup could contain other colgroups (and a
rowgroup other rowgroups).  If this has been rejected, I would like to
see at least a brief summary of the reasoning behind the limit in that
section of the spec.