There are two possibilities that make DocBook really interesting. One is multi-mode rendering and the other is searchable documentation databases.
Multi-mode rendering is the easier, nearer-term possibility; it's the ability to write a document in a single master format that can be rendered in many different display modes (in particular, as both HTML for on-line viewing and as Postscript for high-quality printed output). This capability is pretty well implemented now.
Searchable documentation databases is shorthand for the possibility that DocBook might help get us to a world in which all the documentation on your open-source operating system is one rich, searchable, cross-indexed and hyperlinked database (rather than being scattered across several different formats in multiple locations as it is now).
Ideally, whenever you install a software package on your machine it would register its DocBook documentation into your system's catalog. HTML, properly indexed and cross-linked to the HTML in the rest of your catalog, would be generated. The new package's documentation would then be available through your browser. All your documentation would be searchable through an interface resembling a good Web search engine.
HTML itself is not quite rich enough a format to get us to that world. To name just one lack, you can't explicitly declare index entries in HTML. DocBook does have the semantic richness to support structured documentation databases. Fundamentally that's why so many projects are adopting it.
DocBook has the vices that go with its virtues. Some people find it unpleasantly heavyweight, and too verbose to be really comfortable as a composition format. That's OK; as long as the markup tools they like (things like Perl POD or GNU Texinfo) can generate DocBook out their back ends, we can all still get what we want. It doesn't matter whether or not everybody writes in DocBook — as long as it becomes the common document interchange format that everyone uses, we'll still get unified searchable documentation databases.