Is the concept of Element Equivalence Classes really needed?

Hi,

I think it would be more intuitive if you could specify an occurence
of a class of elements by specifying just the allowed type, like in:

<type name="Foo">
  ...
</type>

<element name="bar">
  <type>
    <element type="Foo"/>
  </type>
</element>

<element name="foo" type="Foo"/>
<element name="baz" type="Foo"/>

In this (experimental) example both foo and baz would be allowed as
a child to bar.

Why do we need equivalence classes to accomplish this? To me
they just add an unnecessary level of abstraction. What am I missing?

Cheers

David Rosenborg
OM Technology AB