I totally agree with you on the issue of root development, the health of the plant and eventual yield will depend primarily on healthy root development, and the plants ability to absorb and process nutes, but the reason I "double transplant" is mainly for practical purposes and to save on resources. I clone in 2" rockwool cubes. Once I see healthy root development extending an inch or so outside the cubes, I put them in 4.5" pots with ProMix, because I can remove the humidomes from my clone rack, and continue to veg the new little babies under my T8's without having to fire up those 4 1000w MH's just yet. Do the quick math on 1 week of 4 T8's as opposed to the 1000w MH's. I get 20 under each 4' shoplight with room to spare. Then I go from there to the 1 gals. and under the MH's, and I go 6 pots by 6 pots under 1 1000w MH for 10-14 days, my clones are usually at my preferred height by then (12"-18") to start flowering. THEN I transplant them to 5 gal. deep pots for flowering for 8-10 weeks. Then, and only then, do I have to start spending all the resources to fire those 4 1000W HPS's, with my plants in a 3 pot by 3 pot configuration under each light, and have to start dealing with the heat issues, cost, etc...
I've done it this way for MANY years, and with a decent indica strain, with everything dialed in, I have managed to pull consistantly almost 1.2 grams per watt of light utilized during flowering...oh yeah...and that's DRY weight! That's over 2lbs. per 1000w light, and those are real world, time proven, not hypothetical numbers, so though I may still have a bit of learning and fine-tuning to do, I gotta stick with what works for me. Yeah, granted, it's a little more work, but I don't mind getting my fingers a little dirty from time to time, I just turn on the radio and chill, very cheap therapy, and still seems like a noble thought, drawing your sustanence from the soil, if it was good enough for my forefathers, it's good enough for me!
Also, yes, I realize that we are dealing with a "weed", that has grown under some pretty adverse conditions for eons without the "help" of man, but I love this plant, and love the whole science and art behind growin' that perfect nug! So I don't mind at all puttin' in the time that it takes to do it...beats the hell out of hangin' out in bars or spendin' all my free time on "FaceBook"!
Thanks again for EVERYONE'S input. whether I ultimately agree with it or not, I do appreciate it, and that's how I continue to learn....