Freeing the ED chroot from the image brings a quite noticeable performance boost, because it eliminates the problem of having one file system on top of another, both not necessarily aligned to each other.

But it is not necessary to have a dedicated partition for that chroot. All it needs is a folder on a file system that supports POSIX file permissions (like the ext filesystems but not vfat).
Basically I could release ED "images" as simple tarballs containing only directories and files that users simply extract into a dedicated directory. The only reason I don't do this is because I assume that most people still have their big partition as vfat MyDocs and therefore require some POSIX-compatible overlay.

I for one have three different chroots running at the moment: wheezy_armel, wheezy_armhf and jessie_armel
Each of them sits in its own subdirectory (with these names) on my /home/user partition.