Here are two simple use cases where wikis and weblogs could be used to glue together complex apps:
Pipejects: producer writes requests into a weblog, consumer subscribes to it, and writes responses into another weblog.
For example, producer writes arbitrary pages into a weblog, and consumer writes only those that match a query string into its responses weblog.
For such scenarios, an instant-messaging standard for feed updates would be nice.
Plugjects: producer publishes data in wiki, consumer subscribes to it, and writes additional data into another wiki, correlated by wiki names.
For example, the producer wiki is a central enterprise wiki, and many consumers subscribe to it, and provide additional information about the pages (for example extracted information) in their output wiki.
The central wiki could then again show this additional information on its pages. This is a nice example of the social software collage approach, with the benefits of identified contributors, data ownership, and no shared state.