Depends what you want it to be necessary for.
Is it necessary in order to produce a third party product? No, but it makes life a hell of a lot easier.
Is it necessary in order to create a thriving third party market? I'd argue yes. Without that easy option, a thriving 3PP market will not exist. There was one with the OGL in the 3E days; there is a much, much smaller one in the GSL days; and there would be an even smaller one without either.
Is it necessary in order to produce a third party product? No, but it makes life a hell of a lot easier.
Is it necessary in order to create a thriving third party market? I'd argue yes. Without that easy option, a thriving 3PP market will not exist. There was one with the OGL in the 3E days; there is a much, much smaller one in the GSL days; and there would be an even smaller one without either.