I could be missing something, and maybe it's my turn to be obtuse today, but why would it not be manifestly superior to train a plant in such a way that a maximum amount of light is reaching a maximum number of bud-sites? Is the assumption that - all else being equal - the plant will yield what it will yield, and that all training accomplishes is a minimum of larf?
In some ways, yeah that's pretty much it. And it's not as silly as it sounds.
The main benefit of training is to ensure no light is wasted on areas where there is no part of the plant (or plants) growing. The other advantage is to produce an even canopy that receives the optimal amount of light - not more (light stress), and not less (sub-optimal).
If you fill your grow room with clones that all grow to the same height and produce a full, even canopy (SOG, for example), then training/pruning has no benefit at all.
If you grow a single plant that likes to branch out evenly, then again similar result (perhaps with a minimum of training). Or you might have a strain that likes to branch evenly after being topped. Or you might have a strain that takes a lot of pruning and training to reach all the above . . . but might not need any pruning or training if you decide to hang you HID vertically and grow using side-lighting.
Tall, spindly sativas are a good example of this and benefit greatly from side-lighting or vertical growing without any pruning or training at all (I did it for over a decade). The plant always grows toward the light, and diverts energy away from areas that receive little or no light (which die off). That's why you don't need to turn your plants in a vertical grow (been there, done that - no benefit!).
If, on the other hand, we all do what canadian1969 is suggesting and veg our plants for an extended time before putting them in the flowering room, then any undergrowth that receives little or no light from canopy penetration will simply die off anyway - which is a complete waste of energy used to veg it in the first place!
The moral of the story is: there is an optimal amount of surface light for any given space; there is an optimal canopy depth for any given strain; there are optimal growing conditions that will take advantage of both aspects and include such things as temperature, humidity, root zone efficiency (oxygenation, nutrient uptake) and all the other factors we consider in growing (pests, disease, mold, stress factors etc).
It's swings and roundabouts - plants grow where there is light, and die where there is none (in simple terms).
