Um, yes you do see farmers taking the leaves off their plants. Its called harvest. Ask anyone that has grown brassicas why it is important to "pick the leaves" even if its just going to compost.
In addition to reallocating energy from foliage to fruit production, defoliation in cannabis also sends important hormonal signals to the plant that increase fruit size and potency and speed.
Defoliation should be broken down into two categories: indoor and outdoor.
Outdoor: defoliation is less important for light concerns as sunlight penetrates to the ground. Foliage helps protect plant from elements, creates suction (larger plants need more suction to deliver water and nutrients) and aid in respiration. Still, fairly liberal defoil should be considered, especially in the epicenter of the plant. But sunlight is harsh...
Indoor: foliage is largely extraneous for reproduction. The elements are under your control. Plants size reduces the imperative for suction. Artificial light does not penetrate the way sunlight does (not even close), so the imperative to facilitate light penetration becomes the driving imperative. The luxury of controlling the elements means the grower can manipulate the focus of the plant's energy to grower's designs (lots of potent fruit).
There is some validity to the gradual method of defoliation. However, the "recovery time" gradual defoliators constantly site as a reason for gradual defoliation can be completely mitigated by simply upping the plant's micronutrient regimen immediately following comprehensive defoliation.
All-at-once comprehensive defoliation is one of the most powerful methods for robust harvests, if not the single most powerful method. Defoliation can't kill the plant. Period. The only potentially "negative" side effect is that plant stores micronutrients in foliage. Easily accounted for in feeding regimen, and even if it wasn't, plants would not suffer. Grower however would suffer a slightly longer flowering period, say, 7 days.
Here's a timelapse of a comprehensive defoil:
