One key point made is that RNG is much easier to grasp than XML Schema. Witness this short tutorial which tells you most of what you need to know.
Something I haven't fully understood yet is why RNG fans consider PSVI to be a bad idea. As far as I see, any tool which is a general purpose parser for typed XML is going to have to define a metamodel describing the allowable range of documents. Doesn't it make sense to have a standard model for this?
Another thing I don't see is whether and how RNG supports the constrained extensibility of schemas (i.e. subclassing). This is responsible for a fair bit of the complexity in XML Schema, but is essential in some uses (such as GML, which is really a schema framework for defining other concrete schemas).
Probably the most appealing aspect of RNG is its compact syntax. It's much more readable than XML. (
This post gives a rebuttal to the RNG-over-XSD argument. He seems to think that RNG lacks subclassing as well, so maybe that answers one of my questions above.