Microcontent and Feeds
Ken brings to light an issue that will become more and more important as we introduce addition content types (namespace extensions) to our syndicated feeds. Since "feeds" are temporal entities that really serve the purpose of notification, putting information in the feed that will wink out of existence once enough new items have been pushed on the stack is not the way to go if that information has value in its permanence. Put another way, don't put stuff exclusively in the feed that has value outside the feed.
That makes sense if you equate RSS (and its varients) with a "notification feed", and in fact Ken provides some really good suggestions for creating "microcontent" and linking to that from the different syncdication formats.
I think what we're seeing, however, is that RSS evolving beyond being simply a feed format, and is instead becoming more of a standard for content representation. You can argue that this is a misuse of the format, but there seems to be a great deal of innovation going on related to the <item> element even taken out of the context of the <channel> element. Why would that be? My guess is because it's an easy starting point, with common meta-information that almost any kind of content would require, AND because RSS has promoted the use of XML namespaces and provided enough examples to make extensions accessible.
So anyway, I don't think there's a right or wrong answer here, but take Ken's advice and make sure that any information you're publishing that has value in its permanence doesn't get washed away in the feed.