It took me a while to find out that with ubuntu-one I can sync only folders in my home-folder. On all other folders the ubuntu-one - option is available in preferences, but the actual actions are greyed out.

The ubuntu-faq is quite clear on that: https://wiki.ubuntu.com/UbuntuOne/FAQ/CanISyncAFolderOutsideMyHomeFolder

But I actually wonder why and if this is going to change and if there is a trick around it (an other one than setting my home to /)

I personally dont have any important data in my home-folder other than the program-configs. All documents, pictures, music are on a folder called /data that is actually on a different partition. That makes it much easier when one wants to reinstall ubuntu.