Number two is that -- contrary to popular belief -- you don't need a stellar QB to win a Super Bowl. In three of the last 9 Super Bowls, the winning QB had a poorer QB rating than the average QB in the league.
For four of the remaining six Super Bowl winners, the QB rating was below 90. Only Peyton Manning in 2006 and Tom Brady in 2004 had QB ratings above 90.
Third, a very poor QB (defined as a QB with a rating well below average) can win you a Super Bowl only if you have other areas of the team that play at a very high level. For example, the past year's Steelers, with Ben Roethlisberger performing below average in terms of QB rating, had the best defense in the league. And of course the 2000 Baltimore Ravens, with Trent Dilfer, had an elite defense and a very good running game.