You know this can be done on top when you have demand for this? I prefer a non-bloated protocol format over XML anytime. How often does the DTD not matter at all ? How often is the encoding fixed by convention ? ...
I use schemas (in the form of RelaxNG most of the time) almost every time I deal with XML. Together with schematron you can make very complex lint-like scripts to verify your data. Actually I program in XML with a self created programming language (formulated in XML). This together with RelaxNG and a good XML makes it fun writing XML and absolutely (syntax-)error free.