How do you have a team captain when there are no teams, BeeJayBee?
QI's format only works properly if contestants are prepared to be 'shown up' when they provide the 'obvious' (but incorrect) answer to a question. Alan Davies's role is to ensure that someone will always do this. (He's actually very intelligent. It's a mark of that intelligence that he recognises when providing an incorrect answer will provide better entertainment than offering a correct one, or simply keeping quiet).
While John Sessions is my favourite actor (who is long overdue for a knighthood), and an ideal guest for QI, the show wouldn't work as well if everyone on it was so knowledgeable and competitive as he is. The regular presence of Alan Davies ensures that, irrespective of who the guests are, there will always be someone who is prepared to 'put their foot in it' with an incorrect answer.
Chris