in XML schemas, can ALL groups be combined?

The standard says:

Schema Component Constraint: All Group Limited
When a model group has {compositor}
<http://www.altova.com/specs_schema1.html>  all all of the following
must be true:
1 one of the following must be true:
1.1 It appears as the model group of a model group definition.
1.2 It appears in a particle with {min occurs}
<http://www.altova.com/specs_schema1.html> ={max occurs}
<http://www.altova.com/specs_schema1.html> =1, and that particle must be
part of a pair which constitutes the {content type}
<http://www.altova.com/specs_schema1.html>  of a complex type
definition.
2 The {max occurs} <http://www.altova.com/specs_schema1.html>  of all
the particles in the {particles}
<http://www.altova.com/specs_schema1.html>  of the group must be 0 or 1.

It's clear from this that you can't have more than one ALL model in a
group. However, most validators interpret it to mean that you can not
have have a group with more than one sub-group that are themselves
internally based on an ALL model. 

This is bad, because it effectively destroys the re-usability of groups,
voiding their whole reason for existence.

Is this interpretation correct? If so, what is the justification?
  /glen 
  
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