Ha! I had the same thought - to bring a scale that's portable to the trail head on group trips so that people could weigh their packs...
My thought, though, was that you could use it to make people bring less. If they knew how much their packs actually weighed, you could try to talk them in to leaving some things behind BEFORE you start out.
That would have been enormously useful on my first backpacking trip ever. I went out on a "beginners" trip with a "seasoned" backpacker from my outdoor adventure club to learn the ropes. I didn't have hardly ANY of my own gear and so I borrowed a lot of his gear. Thank goodness I had at least some of my own - the tent he had intended us to bring was a 12 lb two-person tent (no, I'm not kidding), and he was all cheerful about it - "It's much more manageable when you split the load - only 6 lbs each!" HA! My MSR Hubba Hubba was a little better at 4 lbs, which I thought at the time was "lightweight" (I now own a Tarptent Cloudburst 2, at 2 lbs 6 oz).
Needless to say I ended up carrying a pack that weighed about 40 lbs (lots of it was "group" gear, plus I was carrying unnecessary stuff, that being my first time, my idea of "lightweight" was not exactly solid). All the while he insisted that my pack wasn't much heavier than 25 lbs.
'Course, then he weighed it when he got home. It was nearly at 40 lbs even without my clothing or other gear in it... I'm 5'4" and weigh 115 lbs, mind you.
I converted to UL packing.
Moral of the story: trail-head weighing can be a very good thing. Some people have NO concept of how much things weigh. Sometimes base weight can't be reduced, if you're with someone who already packs minimally, but just has "heavy" gear, for example. But there was a lot we could have left behind, and a scale would have come in mighty handy to demonstrate to the "seasoned" packer that we REALLY needed to ditch some stuff. It was his word against mine that the pack was way too heavy, and I was new - so I sort of deferred. I assumed he knew what he was talking about, and I didn't want to be a sissy...