I can understand why people prefer co2 or pumps. But I really don't understand why, if someone goes to the bother of carrying a pump, they would also go to the expense, bother and weight of carrying co2 also? I understand the old 'belt and suspenders' idea, but I really don't see how it makes any sense to carry both on a bike ride.

I'm a pump guy myself, although I've never needed to pump more than twice - so I based on my own experience I could get away with CO2 w/ just a couple cartridges.