Yeah, you can keep on singing that tune until spider mites, mold spores, aphids, etc get sucked into your room and your harvest goes to shit. Your co2 doesn't get replaced for free...you have to run that carbon filter 24/7 which costs $ plus the carbon will be spent faster. On the other hand, I spend maybe..$8 a month to maintain 1500 PPM of co2 in my room, have no pathway for pests to enter and have COMPLETE control of daytime and night time temperatures and humidity.
So I'm achieving around 30% more harvest weight, higher overall quality and density, and less chance for disease and bug outbreaks for $8 a month. And that Floats my boat...big time