not an expert but i guess size and cost. You can have reduction is space. You have to consider also max temp to make CO2 work at its best and cycles/years to make the best use out of it. Electricity cost may be a plus also. Look into side effects of CO2 if ur grow is in your apartment.
Nug of the Month: Oct 2014 - Plant of the Month: Nov 214
I agree with C but it does help to introduce Co2 into a tent a couple times a day. There is no reason for you to install a Co2 generator in a tent its way overkill. If anything buy a exhale bag. Those work really well in tents.
i use sugar, yeast and water in a soda bottle with a pipe to my fan so that the fan blows it on the top of the plants i think it works you change it every two weeks i have four bottles right now (4 kg of sugar in 4 packages of yeast in 4 two litre bottles on two fans one on the tops and one from down up)