I think there's been a similar topic before and it's a question which comes up repeatedly.
On supermarkets having their own standards; this is annoying, yes but I can understand it if you think about parallels with your own business. Imagine you've had a recall or a near miss in one factory and realise there is a gap in the FSQMS at all sites which isn't currently audited? You would change the FSQMS at all sites and audit it right? Now imagine you'd have to wait for that change for a couple of years and when you do ask for that change it gets watered down by other decision makers... That's why the supermarkets have their own standards.
The other point on quality of auditing is, in my opinion, a valid one. I've never come across a BRC auditor who was as thorough as I'd want them to be. There is in general a lack of tenacity and really driving into a system when non conformity (or a hint of it) is seen. That said, in general, establishments who pass a BRC audit are, in general making safe food IMO so in that respect irrespective of whether the auditing is perfect, it's working. You will always get variation in auditor quality, just as you get variation in Technical Manager quality. One solution, albeit an expensive one would be to have two auditors. Two people are much less likely to miss something or get swayed by impressive presentation and "stage management" than one (we all know that factories do it people.)
The real value of BRC to me is earlier in the supply chain. As I said previously, I feel fairly confident that factories passing BRC are making safe food. Although in some higher risk ingredients you still may decide to audit yourself, having BRC certification for lower risk ingredients is helpful I think.