First off, nobody really knows how good a backup QB is until he plays, so this list is an exercise in futility from the get-go.
Secondly, evaluating QB groups should put a massive emphasis on the starter, since most of the time he'll take the vast majority of the snaps. QB depth matters, but it's not nearly as important as having a good QB in the first place.
According to this list, Kirwan seems to think that a better QB group is one that is pretty much guaranteed to be 'good to decent', but has no chance of being 'great'. Which is absolutely stupid, because you don't win SBs with good QBs. You win it with exceptional ones.
Eagles: They have superior depth, but their ceiling just isn't that high. Feeley and McNabb and Vick range from half-decent to pretty good to good, but the fact is that they top out at 'good', because McNabb's their best QB and even 100% healthy he isn't anywhere near the player that Brady, Brees, and Manning are.
Cardinals: a QB who's pushing 40 and has a track record of being unable to make it late into the season, backed up by a former top-10 pick who can't win the starting job even though multiple coaches have tried to hand it to him? We still have no idea if Leinart's any good, but because he won a Heisman a few years ago that makes Arizona's QB group better than New England's?
Cowboys: Same argument as Philly's, although I'd argue that the Cowboys are in a better position than Philly is.
Chargers: Same as Philly again.
Saints: Mark Brunell and Joey Harrington are considered good backups? Really?
Basically, I get the assumptions that this whole exercise is based on, and the rankings are internally consistent based on that, but I would just argue that it's a flawed and ridiculous premise. He's ranking QB depth, not overall group rating, and those terms are far from interchangable.
here isn't a team in the league that wouldn't trade its quarterbacks for the Patriots', because the Pats have the best QB group, followed by the Saints and Colts. Why? Because there are two things that matter: 1) what's the ceiling of your QBs (how good is your starter, basically) and 2) how good is your depth? Kirwan completely ignored the first half of the equation, despite the fact that it's significantly more important.