I think all the judges are given roles - the nasty one, the woman who goes weak at the knees at the men, the purist old school traditionalist, and the excitable one. We don't know what happens when any of the men change, but when the woman changes she always becomes 'the woman who goes weak at the knees at the men' - that can't all just be natural. So whether Karen or Anton or whoever would work as a judge would depend what role they were replacing. I think Anton could replace Len as the traditionalist, and we'd see a different side of him. I think Karen would be annoying as the woman who goes weak at the knees at the men. If there were two women and Karen was given a different role to Darcy, like the traditionalist, that might work better.
Was it me or were all the dresses looking a bit ropey? Perhaps everyone backstage is exhausted after Blackpool.