There are many reasons to register now!
The "usability" in "World Usability Day" has become misleading. The WUD is about ALL topics that have something to do with (digital) product development. Of course, user experience aspects play an important role, but much also revolves around (product) and corporate culture, self-organization, ethics and everyday work in the digital industry.
It is therefore always worthwhile to be there. Of course also as a big family reunion :-)