Having rdf: elements without declaring the namespace URI for this prefix is obviously a bug. We must either drop the elements or add the declaration. By the way the declaration may reside right in the rdf:metadata element, it doesn't have to necessarily be at the top level.
Generally, "plain SVG" makes sense in two situations: when you need to save some space, or when you have to deal with some old non-namespace-aware XML technology. The first consideration is legitimate, but as for the second, the best you can do is upgrade. In fact, I know of only one such technology: DTD, and it's really better left to die a peaceful death by now. XML Schema, Schematron, and other new schema languages are so much more powerful, and they won't have any problem with foreign-namespace elements.