Anotherthing,
That's what we do at LL. On a watch of ten or eleven ATCOs, we might have five or more qualified Supervisors, so usually each of the core MMAA shifts are Sup'ed by a different ATCO. It really spreads the workload, and means that all the 'supervisors' spend most of their time wearing headsets.
IMHO being a supervisor (at LL anyway) does make one a better controller, and of course there's usually more than one qualified Sup working with a headset if the rostered Sup wants to get advice or guidance.