My guess is that each strain has a 'sweet spot' of proper light schedule and spectrum, but 18/6 during the growth phase is a good starting point. I much prefer using the 18/6 growth schedule, but there are times when 24/0 is a necessary evil.

Like if there is a problem with other lights being turned on and interupting the dark period, (bathroom light, kitchen light...) or if you can't afford a timer...24/0 is a good temporary option until you can rectify the situation.

Although the lights can provide overnight heat for the little darlings, it can be tricky keeping them hydrated properly, and they just don't perform as well under 24/0. At least not in my experience.

I'm not even positive that 24/0 is a good idea for autoflower strains, but I haven't grown any of those yet.