I suspect the use case for this feature was being able to set an explicit value in one block of meta-attributes, then have another rule-based block of attributes that resets the value to the default. Simply omitting the meta-attribute from the rule-based block wouldn't cause the explicit value to be removed.

In T800#11838, @nrwahl2 wrote:

<define name="non-empty-string">
   <data type="string">
      <except><value></value></except>
   </data>
</define>

It turns out that due to the same normalization you described, any value that consists of only spaces is rejected. Not just empty strings.

Which might be fine for our purposes, but we need to be aware of it.