For 3.6.0 The NominalDiameter element was updated to meet DEXPI requirements but so far no-one apart from AVEVA is using it. Implementations and the example file are all using a GenericAttribute. Is there a reason for this?? NominalDiameter is an important property that should in my opinion warrant use of its encoding using the NominalDiameter element.

The DEXPI verificator, which is working on DEXPI Specification 0.9, does only verify the generic Attributes according to the DEXPI specification.
If a nominal diameter is given in other attributes than a generic attribute, the verificator does not check those. Because of that, the DEXPI example file only provides the nominal diameter as a generic attribute. If you want to use the native nominal diameter attribute, you are free to do so.