Ben makes a good, valid point. Reminds me of the 'Mark as Junk' and 'Mark as Not Junk' icons holding hands together on the Mailer toolbar.
Under the hood, it appears to be a limitation with BonoboUI (or perhaps just how it is being used ?) that if you hide buttons based on context [like message (not) marked junk, HTML/Plain text etc.] , you mess up their placements in the toolbar and they are free to jump left and right to their current positions in different contexts.
Think Dynamic Menu generation. One other reason to take that decisive step towards gtktoolbar.