Substitution Groups - Use 'em or lose 'em?

All,

 

We are working on extensibility models for standardized business 
exchange
schemas. These schemas, under development by several standards 
organizations
make extensive use of hierarchical schemas and namespaces, some 
of which
include schemas developed by third parties.

When a user or 
user community seeks to use these schemas, and, needs to
modify them in some 
way (without altering the underlying standardized
schemas), substitution 
groups can be a powerful and explicit mechanism for
such extensions and 
restrictions.

Several participants in these standard schema efforts have 
expressed reserve
from utilizing the W3C mechanism of substitution groups due 
to their
experience with non-uniform support of parsers for this schema 
feature.

In your experience, what are the concerns or recommendations on 
the
incorporation of substitution groups into the naming and design rules 
of
standardized schemas?

Should substitution groups be relied upon as 
an extension mechanism?

Marty Burns
Hypertek, Inc.
P 
+1(301)315-9101
E