The only reason people do that is for a quick harvest. To save a couple weeks which IMO is very impatient. I'd go with an auto instead seeing as how they'll have more light during veg which means more growth, with the added benefit of a quick change to flowering as opposed to a plant on 12/12 which will receive less light during veg. I guess 18/6 isn't an option if you're so impatient (not saying you) that you can't wait an extra two weeks after switching to 12/12 for it to finally start flowering.