A problem is that most of the intellectuals they get in are from the Oxbridge brigade. I'm not saying that is a problem in itself but maybe it is a consious decision not to overload the program with them and have a broader range of guests.
Saying that, David Mitchell seems to not go the same route as McGrath and just has a swearing rant. It seems to have become his speciallity over the last few years.
I'm also wondering if it is a consious desicion not to ask in people from other shows that could be good for QI.
Ian Hislop, Paul Merton, Will Self, Jack Dee, Robert Webb, Dr Phil Hammond etc. Ofcourse, it could be that they have been asked and don't want to appear on QI.
I was watching a couple of programs the other day and thinking about QI guests and up popped two people who could quit the show perfectly.
Michael Palin and Sir David Attenborough. Slightly leftfield for Sir Dave but after seeing him interviewed he has very good anecdotes, great knowledge and, I feel, a good sense of humour.