Show me 2 plants, from the same mother, one in 24/0 and one in 18/6, and show me a big difference between them. My opinion is exactly that, an opinion, but I have tried both time periods on different grows. The only difference I find, is that the plants grow more. Not a lot, but more. Now, I havent looked at the roots when in the veg (but in the end at harvest, the roots are about the same size), but if the canopy is larger in a plant with 24 rather then 18, and its healthy, then it probably also has a root system to support that canopy. Plant is bigger = bigger root system.

That being said...I use 20/4 or 18/6, but not for the reasons of better root development or something like that. I use it because I believe there are more advantages to having some darkness then the advantages of light 24/7, even though I believe plants need light far more then they ever need darkness (veg state of course).

The advantages of 18/6 or something similar that I see, is that you greatly enhance your bulb's life, you wont need to water as much, your energy bill will be substantially lower if you have a lot of lights, you will have an easier time keeping things cool. I use 24/0 when I am vegging from clone for 1 week and then going into flowering.