Removing multiple-hierarchy support would be an amazingly bad idea. It would force system administrators to put all their tasks into one giant rigid hierarchy, removing all flexibility. It would also make a lot of configurations impossible.
For example, let's say I have a simple system with 3 processes: httpd, sshd, and xeyes.
I want the following constraints:
httpd: limit network, limit memory, don't freeze
sshd: do freeze
xeyes: limit memory, do freeze
Assuming there are three cgroups-- network, memory, and freezer-- how can I create a single hierarchy that will do what I want? Unless I am somehow misinterpreting what is being proposed here, this would be impossible if multiple-hierarchy support was removed.
(And yes, I know i could use the old rlimit stuff. But there are presumably other things that cgroups can do that rlimit can't.)