Smoking a huge quantity just isn't worth it for me anymore. Hell, it probably never was but I was inexperienced and didn't have any tolerance so it was sort of a different scenario. When I first started smoking, it was EASY to go through an 8th, which obviously is nothing compared to the quantities being mentioned in this thread. The quality I was getting back then probably also sucked compared to now.
But after being a daily smoker for years, tolerance has a way of building up and once the seasoned smoker hits his peak (even if it's only after a few hits) that's about all you're going to get. Sure, you'll get increasingly monged/spaced out which is KIND of like being high, but for me, if I'm not feeling myself getting actively higher anymore, it's just a waste of weed.