From the perspective of someone who runs a field and events at that field, CO2 is a pain when compared to HPA. It takes longer to fill, requires staff, requires more pre-planning (is the bulk tank full or do we need to get another?) and can over-pressurize if you start the day at 35* and end the day at 75*.
Just my opinion.
That said, I have no problem with 12ies and really could care less what gas you use if you bring it yourself. That's D-Day's stance this year. They haven't banned the use of CO2, they just won't be offering to fill it. You'll have to bring your own if you want to use it.