The idea is that any SingleSelectionModel can be thought of as a set that contains either zero or one items. So it seems appropriate that it should have the new methods - and after all, the whole point is to be able to treat them uniformly.
SetSelectionModel isn't intended to imply that the SelectionModel can contain multiple items; if that's unclear, perhaps it needs to be clarified somehow? http://gwt-code-reviews.appspot.com/1798806/ -- http://groups.google.com/group/Google-Web-Toolkit-Contributors
