Cause it's more work for you? I actually agree with you (since most people can't grow ridiculous trees in solo cups like fdd)...if you're going for ridiculous yield, use at least 10 gallon buckets or bags. I've also heard of people using burlap sacks because the roots will grow through it...that way you have some nice fertilized dirt, but also endless dirt in the ground for roots to extend into. I've never seen anybody do that here, so I don't really know the results.
Plus, he mentioned using fox farms. That stuff is not cheap...getting 15 gallons per plant is gonna add up reaaal fast.