First; As a professional trail builder I beg you all to refrain from skidding your back wheel, It damages trails.
Second; I believe the biggest braking problem you have over there is that you've got the levers backwards. If you had them the right way round, like here in Canada, you wouldn't have nearly as much trouble trying to stop.
Third; Sheldon was the worst MTB'r and never rode suspension forks.
On roads it's easy to modulate a properly set-up bike to the point where you can decide how much slip the rear tyre gets, but on uneven terrain it just isn't that easy. The worse the terrain gets, the more difficult modulation becomes. Eventually it becomes an "all or nothing" affair.
As a rider the logical response is to just lock them up. No matter how good you try to modulate the rear brake, once the tyre locks up momentarily any non-professional rider will instinctively keep the brake on hard because you just can't regain proper braking that easily. Attempting to regain control over the brake would mean letting the brake go for a bit and getting a feel for the tyre's grip again, which will add several yards to your braking distance.
I completely understand what you say about damaging the trials though.
As for the second point, it's hard to agree. IMO it depends on which hand you have the best control over. I'm left handed, so I should be driving the UK layout instead of having them the proper way around like I do now.