I have the valve on my HP-BH and I do use it, I can open and close very easily with my third finger while working, I'm usually switching between distances as I work and it becomes tedious constantly changing my regulator, so it's a real bonus, that's the upside, the downside is that I have more than once hit by accident and unwillingly turned it up, that's what happened to my failed Jessica pic, I was working on the lips and blasted a huge dot on the left of her bottom lip as well as almost filling in the background, for that reason I would much prefer an external mac that is further under the brush.
The cut away you mentioned, it's only purpose is allow to pull the needle chuck all the way back in jiffy to blast out a clog, I've never used it, aside fro that it just maybe looks cool, same as the adjustable needle travel screw, I never use that either, if I want less paint in relation to air, I just pull back less, that means I don't have to stop to adjust the thing, but that's where all your practice helps out.
