XML is, frankly, overkill for my needs. It permits the description of arbitrarily-complex data, but I don't really have any need for that level of power. It's syntactically a bit of a nuisance -- not a huge problem, but not really designed to clearly represent simple key/value pairs like what I'm looking for here. And while it is a highly standardized data description language, there is no equally-standard query language. There are many query languages I could have adopted, but again, all are pretty ornate.
I contemplated going with the standard, but decided it would be a nuisance. And, frankly, much of the point of Wiki is to eliminate that particular nuisance -- Wiki is intentionally a highly simplified version of HTML. So in the same spirit, I decided to create a highly simplified data-representation system. It can't do everything -- indeed, at this point its capabilities are quite limited -- but it's very quick and easy to use, and powerful within the intended domains.
So while the standards geek in me is slightly burned by it, I think it's more practical to reinvent the wheel yet again, to come up with something that suits the problem at hand.