RE: XML versus Relational Database

I guess I don't quite understand the statement that "XML vs Relational DB is a non-issue". If your needs are extremely data-centric and you don't much care about extensibility- the info you store does not change structure- then I suppose there is truth to it. However, for heterogeneous stores of information from multiple sources, many of whom, despite best efforts, may not conform to a standardized schema and yet may be providing semantically equivalent data, it seems to me that an RDBMS solution, with or without embedded XSLT, is pretty much inadequate, mostly due to the need for metadata predefinition in an RDBMS and for performance issues around transformation.

XML allows metadata and data to be treated symetrically, with context dependent entirely on each query. One man's metadata is, after all, another man's data. RDBMs, by their very nature, strike me as quite limited in this regard. What am I missing?