> I'm looking for concrete examples.  Why should I want to create
> instances with mixed MODS and MADS namespaces?

You wouldn't.  We're talking about instance with mixed mods and mstl
namespaces, or with mixed mads and mstl namespaces, but (normally) never
more than two namespaces, default plus mstl.

Sorry, the type library discussion got sidetracked by the question of  a
"single namespace" which is a somewhat different issue, leading to
discussion of mixing mods and mads namespaces --  mixing mods and mads
namespaces was only offered as an example of what having a type library
allows you to avoid. And it allows you to avoid the situation where adding a
third schema would mean mixing in a third namespace and so on.

> This is critical issue.  For anyone who's ever worked with instance
> data across multiple namespaces, every time you add a namespace -- in
> the context of XSLT processing, or query, or whatever -- the level of
> complexity goes up significantly.

If  we only need to deal with two namespaces per instance (default plus
mstl) does that help?