Suppose we regard <![CDATA[ ]]> as invalid in element-only content.
If the processor is non-validating, it will not check this, and
will produce valid output from invalid input.
You might for example run a document through such a processor merely
to change its character encoding. It would be unfortunate if this
process changed the document's validity.
Slightly less plausibly, a processor might decide to output all character
data as CDATA sections to avoid using character entities. This process
could make a valid document invalid.
-- Richard