Okay that's a good question. I've answered it elsewhere, but it should be included in this thread. IT depends partially on personal preference and grow style, but my reasons are: -Very easy to stabilize temps when heat from your lights is not a variable over time -I can visit them at any time, day or night, without fear of stress -No timers to buy or have fail on ya -Going from 24/0 to 12/12 is a big heads up to the plant FLOWER NOW! as it is the first time it has ever seen darnkess. That's why I go directly, with no intermediate step. Also, a slick trick for the outdoor crowd is to keep plants on 24/0 until August 1 and then put them outside. They will begin to flower immediately seeing darkness for the first time, and finish before frost.