Ed Dumbill has posted a nice short analysis of why he chose RDF instead of plain old XML for DOAP.

When he's talking about the benefits of using RDF, it's not really anything I hadn't heard before when people are extolling the virtues of RDF; but it really helps to see what those benefits mean in the context of a specific application like DOAP. He wanted a decentralized system for describing projects and the relations between them and the people working on them, recognizing that any one source is unlikely to know all of those things about a project. He wanted the DOAP vocabulary to be easily extensible, not only by established ontologies like Dublin Core and FOAF, but by as-yet-unknown vocabularies.