"I think proscription to have been ultimately harmful ..."

Bollocks! Who did it harm?

It applied to a particular club, at a particular time - a club which had specific objectives and formulated a policy which helped it to meet those objectives.

Having said that, all of the best clubs that I attended - including the first one that I ever attended, in my home town - had policies (usually unwritten) which might be described as "proscriptive". But those policies were about taste and a common understanding of the musical genre that the members of the club (both performers and audience) were interested in and enjoyed. It was NEVER, as the 'everything-is-folk' brigade continue to insist, about compulsion or restriction.