My experience with defoliation is this:
I defoliate throughout my grow, a little bit here, a little bit there. In the early stages of LST I tuck big fan leaves down as far as I can so they aren't blocking light. I even bend large fan leaves stems to get them out of the way. Once I have a little bush with a ton of tucked fan leaves, some of them just have to go. I would like to say by this point I don't really have a choice to do so. If I un tuck the leaves they will completely shade my new branches I worked so hard training to produce.
It's like anything else, there is a time and a place. Do I LST, top, fim, or bend? NO, I do all those things because every empty space in my canopy might require any one to fill, at any particular spot.
I leave on everything I can and take off only what I have to.
I have also had the experience of defoliating causes a growth SPURT. It seams the plants really do use those big fan leaves for something-when I cut them off they seam to go into over drive to produce new growth to compensate for the loss of leaf area.
I couldn't grow plants as big as I do with out defoliating, and that's just the truth-i've tried. My style of creating bushes calls for it.
Sometimes removing a fan leaf will create enough light for a lower branch to finally shoot up. Now instead of a big fan leaf I have a new branch sticking up in my canopy.
If you took two plants, same strain, and let them grow naturally, defoliated one, and left the other one in tact, the OF COURSE the in tact plant would produce more.
When you train your plant the way I do there is a definite benefit to removing fan leaves. There is a time and a place for everything. A lower branch covered by a fan leaf isn't going to make it to the canopy, and your gonna have larf. I'd rather have a cola then a fan leaf any day.
Anyone who says defoliation is counter productive needs to look no further then any orchard/nursery/farm. Training and defoliating are proven ways to increase yeild if you know what your doing and they are implemented correctly.
Look at everyone doing SCROG, they yeild pretty damn good and they do some serious defoliation.