The main reason is to allow the ARIA library to be used independent of the widget library, in case you write an app the uses the low level DOM library but not widgets.
We've been moving away from using client.ui as a catch-all for all UI related things in favor or segregated packages for different features. http://gwt-code-reviews.appspot.com/1624803/ -- http://groups.google.com/group/Google-Web-Toolkit-Contributors
