Adding metadata to Wikibooks would have a couple very beneficial effects in the near-term future. The biggest of these would be increasing the accessibility of Wikibooks’ resources in most major search engines - Schema.org was formed collaboratively by Bing, Google, Yahoo!, and Yandex, with the explicitly stated aim of making it easier for their users to turn up high quality relevant results in their searches. The addition of good metadata to Wikibooks content should improve its accessibility via search engines. It will also improve the preview of Wikibooks content that is shown in search results, since most major search engines use metadata markup to generate better previews where it is available.
As an example of how this could play out, take a look at the Wikijunior book about the solar system - specifically, it’s chapter about the sun. It presents a pretty good overview of the Sun, aimed at elementary school students. Despite the fact that it’s a pretty solid chapter, it doesn’t get very much traffic - only about 220 views a month. This seems like way too little exposure for such a high quality book. After playing with Google for quite a bit, it became obvious that this chapter was not ranked highly on most relevant keywords - the addition of accurate metadata to this book should improve its accessibility via relevant keywords. (You can’t add metadata to low quality pages and expect a meteoric search engine boost, but when you add metadata to pages that already have high quality content, the results can be remarkable.)
We also anticipate that there may be unexpectedly creative uses of HTML Tags to support things other than metadata. Although we cannot guarantee we’ll be able to provide technical support for all such uses, we are certainly excited about them. We’ll support cool side projects where we can, and if something comes up that we can’t support we’ll try to connect you with volunteers with the appropriate skill-sets to progress your project.