XML is fashionable, so it might be a selling point for SMIng. And are they useful even without operations that work on compound types. Compound data types. Put them in at the first go, then make the mapping to a protocol that doesn't know about them an issue, or assume that the underlying protocol can ship them and make that the issue. You need tools.
Tools are there, libsmi.
Libsmi is getting popular. Motivation for using agent caps as an example for the extension mechanism. It is useful to put in some motivating text in the document for this. The SMIng document is a large document. If you compare it to its SMIv2 counterparts including the ASN.1 definition, this is to be expected of course.