I don't know about a comprehensive footprint analysis, but my understanding is that the CO2 for cartridges is captured as a byproduct of industrial combustions and sometimes organic processes. ie, it would otherwise be released into the atmosphere, so the net effect is nothing.

I do recall repairing a flat one night, in freezing rain. It's hard to imagine someone begrudging me the time saved as laziness, not that you'd particularly care in that situation. I've used maybe three of them in the last 5 years so I'd say that it's only a small amount of laziness. Considering that I have probably expended more energy carrying them around (5500 - 7300 miles per year @roadwarrior) than I have saved by not pumping by hand on the couple of flats.