12/12 from seed

Nothing, because most plants need 16-24 hours of light per day for their vegetative stage. I would say that with 18/6 light hours, you generally achieve the most effective light usage in relation to energy costs. Once your plant is mature enough, you can switch to 12/12 (assuming it's a photoperiod plant).. but before that, I think it makes little sense. With auto varieties, it might work, but in my opinion, it will lead to significant losses in yield.

No, it is just wrong and a waste of time. Give the plant what it deserves, a decent, quality life, not a torture test. 18/6 for veg and 12/12 for flowering is the only way to grow a photoperiod plant.