True about plugin_wiki, but not everyone uses it. I know I don't, just because I am mostly developing internal enterprise applications that do CRUD operations on a database. Also, there are other plugins that require jQuery UI, so having those plugins include their own version of it is a bad idea, unless you make plugin_wiki a dependency of every plugin that needs jQuery UI. That was my main motivation for wanting it in the core, was that plugins don't need their own installation of it and you don't have to deal with multiple themes and things like that.
After thinking about this a bit more, maybe it would a good idea to move jQuery UI from plugin_wiki to its own plugin. That way it doesn't add bloat to applications that don't need it. You install the plugin_ui or whatever you want to call it, then other plugins or applications that want to use jQuery UI can just make plugin_ui a requirement without having to pull in the entire functionality of plugin_wiki. Would this be a better solution?
