As I said, YMMV. It happens to me at least twice a year, but I start in an outstation. It usually happens when a flight goes mechanical and would cause me to misconnect. It also happens when CO cancels an entire day's schedule into EWR due to snow (I'm at an ExpressJet/Colgan-only station), but other airlines are still flying mainline jets to my local airport. I've been rerouted onto AC from NW and onto NW and AS from CO within the last twelve months, so twice in about 20 trips. Each time the rerouting saved 24 hours. Maybe it's different if you live at a hub and take mostly nonstops, rather than trying to get between secondary markets (the two above trips were to YXE (Saskatoon, SK) and OAK).

So, I'll continue to do it my way.