Eric: "As real applications appear and crystalize, then schemas can be formalized for those applications."
*Exactly*. RSS-Data can serve as "training wheels" for new extensions.
Got a new or unusual chunk of data you want to expose to the world? Wrap it up as RSS-Data and get it out there where people can immediately see it and play with it in their RSS-Data-aware aggregators. If it turns out that you're on to something, interested parties can sit down and start crafting a formal specification, and once the aggregators starting implementing the new spec, you do away with the RSS-Data wrapper and move on.
Roger Benningfield commented