At present in Workbench you can select either a single branch to show in the history pane, or it will show changesets from all branches.
Also there is an option to filter closed vs. non-closed branches.
I suggest that instead THG allows you to select any set of branches to be shown at one time. For instance we have some projects where individual large or important features are done on their own branch. Periodically branches within a project need to be merged in order to keep things up to date. Or sometimes a graft is needed. There is no way to just isolate the few branches involved without showing up numerous other things that just get in the way.
Note - the option to include or not include closed branches would seem to be unnecessary if this were implemented.