I'm sure it depends totally on what type of control you're doing and how busy the sectors tend to get, but from the point of view of a (stripless) UAC in busy airspace, it would be total chaos without planning controllers. We wouldn't be able to have anywhere near the sector capacities we currently have.
How are you supposed to be able to coordinate with other sectors as well as talk on the frequency with anything more than a couple or few aircarft in your setor?
I do actually have to coordinate with an adjacent sector which has an "on call planner" policy, and am frequently left hanging on the phone waitinng for the controller to finish speaking to the aircraft. Fine if its not urgent and/or you're not busy yourself. But when you have 5 other calls to make, its causing workload for us as well as them.