It is true the Commander has the right to have passengers weighed, but in reality it's never going to happen on your average bucket and spade flight to the Costas.
We now live in an age where most aircraft have good range/payload capability and runways are longer.
When we operated a/c like the B737-200 on sectors such as Leeds to Palma in the 1980s weighing the passengers and being able to carry an extra 250 kg of fuel was the difference between a tech stop for fuel or not.
We would be taxied to Leeds from base prior to the flight so it was a simple matter to ask the handling agent to weigh passengers when they checked in. They were quite used to this request from time to time and would quite happily oblige. Of course, as a footnote, if the actual weight was more than assumed weights you were then legally obliged to use the actual weight.
So King Dong, the necessity to weigh passengers may well have diminished but it is enshrined in statute that the Commander has the absolute right to demand that passengers are weighed. One would have to have a good reason to do so but I would take issue with your statement "but in reality it's never going to happen" - it all depends!
Ps Meant to mention - these were what you would term "bucket and spade" flights.