Question: base-unit vs. unit elements

[C Evans]

Looking at the UCUM XML definition file (ucum-essence <date>.xml), I was surprised to find that the 7 UCUM base units are not defined as elements of type "unit" with a class of "base", but as a separate element type "base-unit".

Is there a discussion anywhere of how this design decision was arrived at?

As base units are themselves usable units (i.e. they are not abstract), it would seem more useful to include them in the set of units, and differentiate them by class. E.g. an XPath expression to return, or count "all defined units", would currently need to include patterns for two separate element types instead of one.