TBH I don't see why people are getting so wound up about it? If it only nets a job or two a year it's worth joining. It doesn't matter if the assessor is good, bad, or indifferent, he or she will have ticked a box. That will give a few customers a bit more confidence.
I think, and it's only a personal opinion, that the flaw in the idea is that it's a manufacturer backed scheme. That will always leave it open to criticism over vested interests. As far as I'm aware there isn't a trade association for plastering? So a better option would be to create one and turn the scheme over to it. That way they will avoid the suspicion that the more you fly the BG flag, or the more BG gear you shift the more favourably you'll be looked on within the scheme.
If it does stay as a BG scheme then I think it'd be a good idea to have membership levels which are based on trade skill levels. So if your work is all in one area of plastering, skimming say, you're level 3. If you work in, or can prove competence in other areas such as lime work or moulded plaster you get a level 2. Not all plasterers are equal, and any skill based scheme should reflect that IMNSHO.