It's all very well weighing bags early in the process, but what about those who load up in the duty-free shop? It is trivial to circumvent a check-in weight, and speaking personally, if I've got a mid-afternoon flight then I may well be carrying lunch with me at check-in. Although it's going to make the flight, it's not going to be in the carry-on bag when I board the aircraft.
What would be far more use is an early size check, on the basis that some of what I see in the boarding queue is really taking the piss when it comes to size. If there has to be a weight check, it needs to be in the run-up to boarding, by which time all stuff to be carried on should have been acquired. It might also make the flight safer, by giving the flight crew a more accurate weight for carry-on.