Identity constraints: complex types with simple content

In 3.11.4 of Schema Part 1, Validation Rule Identity-constraint satisfied,
rule 3 states

For each node in the .target node set. all of the {fields}, with that node
as the context node, evaluate to either an empty node-set or a node-set with
exactly one member, which must have a simple type.


Is it really intended, and is there any good reason, to exclude nodes having
a complex type with simple content?

Michael Kay
http://www.saxonica.com/