I've always thought, and have posted along these lines before, that the forum is effectively giffgaff's shop window. People will judge by what they see. Hopefully people looking at the forum realise that its an open forum, and people can post what they like (as long as they abide by the community guidelines) but I agree that the potential risk is there.
Also there is the potential risk of people looking at the forum and seeing member's frustration and complaints about giffgaff, which are poorly dealt with, e.g. bugs being reported and nothing been done for months, giffgaff staff members failing to come back to threads and answer questions, etc
Its only anecdotal, but I've had potential recruits who didn't join giffgaff, at least partly because of their impressions of the forum. Not saying it was their main reason(s) for not joining, but it played a role