I'm old school and adhere to the old school formula that every bike has a pump, and spare tire (tubulars) or tube. Nobody I know moves a spare tire and jack back and forth among their various cars, yet these same people will own only a single pump for their 3-7 bikes.

But being old school doesn't mean I'm not also a bit lazy, so my commuter bike has a CO2 inflator in the bag and a pump on the frame. When fixing a flat, I use the pump first to take the tire up to "inspection pressure". Once I confirm the tire is properly seated to my satisfaction, I'll use the CO2 to finish the job and bring it to full riding pressure. This saves me time and effort, but bypasses some of the drawbacks of CO2 inflators.