Good question,
It is vital to have headers for the data so that we can link it to the information. In SBRML the model is given explicitly via a model element and later in dimension descriptions the identifiers of the model is being used. In NuML we wanted to be as generic as possible and reduced it to contain just the results.
I can see that an xpath query to nowhere is not really helpful. Perhaps we should use an xlink/xpointer instead? So it would look like
xlink:href="http://somemodelsomewhere.xml#xpointer(id('species_1'))"
if we would restrict ourselves it would look much cleaner than the corresponding path expressions.
Cheers
Frank