Aaron, I agree with you about the weight of Slime tubes, but 25 years of racing road, cyclocross and mountain bikes has taught me that everything is a compromise. For something like my 24H race, the thought of being on the far side of the course at 3am in 40 degree temps trying to repair a tubeless tire issue is not something I care to deal with. For a course like that, there are so many cactus pads and cholla balls that get knocked onto the trail, Slime is pretty standard, despite the weight. The minimal weight of Slime is not going to make or break me but, a burped and hard to remount tire bead could. Besides, the amount of Slime per tube is about the same difference as a light XC tire vs. a heavy XC tire. Not that much.
And you are correct, avoiding pinch flats is all about the right air pressure for the terrain, but then again, what's the right pressure? It's always a fine balancing act to get the lowest amount of pressure to maximize traction and decrease rolling resistance, but not so slight as to invite pinch flats.
This same discussion goes on in the road racing circles. I usually race beefy tires normally saved for training while others race on delicate, hand made tires. A fast handmade tire may never win the race for you, but it can sure lose it for you.