Yes, I know this is an old post, but it is still present for me. I tried to get the modified-hildon-desktop back with Code: apt-get install --reinstall and just got "Package modified-hildon-desktop is not available, but is referred to by another package." and yes, I do have the -Devel/-Testing & CSSU enabled. What's going on in here? I've had this bug since March. Not that I'd really need more than four desktops but I'm just confused, that why I can't browse for nine (and I did check, there's only four visible, no scroller). And yes, I do have the CSSU Transitions tuner installed too.
apt-get install --reinstall