Using Application Dashboard, put some application in the Favorites. Then switch to an alternative launcher (either the default Application Launcher or Application Menu), and switch back to Dashboard. Notice that only the default favorite applications are displayed, any customizations are lost. This is of course not desirable. Why switching to another launcher results in losing such settings as favorite applications? I noticed that the same happens with the favorite applications of Application Menu. I suspect that both Application Dashboard and Application menu use the same "backend". If this is so, they should probably _share_ settings rather than cancelling each other's when switching. I also noticed that the default Application Launcher preserves favorite applications as expected upon switching.
*** This bug has been marked as a duplicate of bug 351770 ***
Conceptually, config in widgets is generally per-instance (you can have as many Application Menus as you want). That Application Launcher preserves favorites is technically the implementation bug here because it's ancient code predating current best practices and has other problems. However, specifically sharing favorites among all launcher instances (with the downside of not being able to have different favorites in different launchers) is under discussion.
I understand what you write perfectly well, but from a user point of view the situation is completely different (and, to my view, unacceptable): I have put a launcher at the lower left corner of my window; then I try another launcher but afterwards I come "back" to the same launcher I was using -- and my favorites are now lost. I have not removed the launcher altogether, I just tried "alternatives". I hope you understand the problem. Furthermore, I would not agree that bug 351770 is a duplicate of this one. Bug 351770 is about sharing favorites between launchers, this one is about preserving favorites when one comes back to a launcher that he has configured before. I added a relevant comment to that bug as well.
^ That's a good point and I think enhances the discussion in the other bug. Since I personally don't think sharing faves between launchers is a good idea either, tying things more directly to the point in time where one is swapped for the other makes more sense to me, too.