First off, Patrick, you omitted one key item. Fans are only invested in their team's backup quarterback if the starter sucks. Quick, name Eli Manning's backup on the Giants. No? How about Tom Brady's? Aaron Rodgers's? Ben Roethlisberger's?
They would be David Carr, Brian Hoyer, Matt Flynn, and the aforementioned Charlie Batch, who actually cracked mediocrity for a couple of seasons with the Lions. That's four guys almost no one has mentioned this year and won't give a second thought to unless one of the elite signal-callers they're backing up goes down or stops winning.
Which brings me to my main point: If you're looking for an American cultural foible to explain our love of the backup QB, it's impatience. We as a country have grown increasingly unwilling to slog through hard times and wait patiently for future success. Whether it's angry voters demanding Barack Obama change things NOW (if not YESTERDAY) or fans scoffing at their favorite band exploring a new musical direction, Americans want results now, and if we can't get them from you we will happily look elsewhere right away.
Maybe it's because we have too much choice, as the digitalization of life gives us endless recourses if something doesn't immediately go as planned, all just a click away. Maybe it's too much modern comfort that's caused to forget that success is a process, often an arduous one. Whatever the reason, football is not immune from our never-ending quest for The Next Best Thing or Something Better Than The Thing We Have Now (or as I like to call it, the iPhone phenomenon). And as long as Daniel Snyder is a football owner and the Marino Curse hangs over Miami, there will be fan bases looking for something better at quarterback.
Am I onto something, Emma? Or is our obsession with backup QBs more prosaic than I've made it out to be?