Comment 1 for bug 820175

They do different things though, the activity thingy lets you switch activities whereas the desktop one makes the current desktop the foreground window. I never liked the desktop plasmoid much, perhaps replacing it with the one that opens the dashboard (i.e. ctrl-f12) would make more sense, then both would have a unified focus on plasma workspaces (i.e. the paradigm that is > boring desktop)