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>