The present windows effect has the option to "Toggle present windows (All desktops)" which can be very handy when you need to quickly find a window on some desktop: you toggle the effect on all desktops and start filtering by typing. Would be nice if overview had this capability.
Present windows has such an option because it doesn't have a desktop bar. IMHO we don't need this option in this effect, however we need to re-evaluate whether the user should be able to navigate between virtual desktops without leaving overview mode by clicking desktop thumbnails.
Merging with bug 448668. *** This bug has been marked as a duplicate of bug 448668 ***
I reported this bug before there was a desktop bar. In any case, one benefit to not leaving overview when navigating desktops is that it makes moving windows from a non-"active" desktop to another much easier. There could be an option to e.g. middle click a thumbnail to immediately activate the desktop, giving users both options.