Gonzo
I've done airfields in my past career and agree with you entirely. I believe that supervising does not only enhance your abilities as a controller (as it gives you more insight into 'the bigger picture'), but that it works the other way as well... If the majority of your roster is sat, bum in seat controlling, with 3 or 4 days a month supervising (instead of the other way round), then I think it will make you a better supervisor.
There are a few substitute supervisors at Swanwick i.e. they do it 2 or 3 times a month to cover a shortfall in manpower - in the main they tend to focus more on the nitty gritty of making the sector work than the normal supervisors do - possibly because as a substitute you have no onerous 'secondary duties' that come with the position... duties that can, if the supervisor allows it, detract fro supervising!
The substitutes get a (very) little extra for doing it, paid only on th edays they actually are rostered to Sup, so it is cost effective.