Problem
Changing workflow states is not intuitive, in fact it's almost indecipherable. Add to this the fact that using the dropbutton as the place where the workflow state change is set exponentially increases the number of transitions needed* and it is clear that this does not scale.
Proposed solution
Showing the states as a button bar makes the transitions transparent to the user. All they need to do is choose the workflow state they want the node to be in when it is saved. The reason for exposing this as a button bar and not as a select list is that it removes a click, exposes the whole flow in one visual and gives the workflow an apparent (if not actual) linearity that makes sense to the author.
*Eg. if I have two states, draft and published, I need two transitions: "save as draft","Save and publish." If I have three, Draft, ready for review, and published, I need nine: "Save as draft","Save draft as ready for review", "Publish draft","Publish reviewed draft","Move from review and save as draft","Unpublish and save as draft", "Unpublish and save as ready for review", "Save and Keep published". And so on.