Page layout allows to set only width or heighth instead of both

In the ODF RelaxNG XML schema of ODF it is possible to set the width or the height of a page, but both attributes are optional.

See http://docs.oasis-open.org/office/v1.2/os/OpenDocument-v1.2-os-part1.html#__RefHeading__1419862_253892949

As for the minimum of the two dimensional page both values are mandatory, I suggest for interoperability of page layout to drop their optional usage and make them for every page layout mandatory.

Or is there a use case known to someone to omit page height / width but using some other value?

Here the existing description from the ODF 1.2 RelaxNG (see http://docs.oasis-open.org/office/v1.2/os/OpenDocument-v1.2-os-schema.rng)
<define name="style-page-layout-properties-attlist">
<interleave>
<optional>
<attribute name="fo:page-width">
<ref name="length"/>
</attribute>
</optional>
<optional>
<attribute name="fo:page-height">
<ref name="length"/>
</attribute>
</optional>