.. provided
(a) you base your "standard weights" on stat samples for a relevant population, and
(b) are applying the standard weights to populations of sufficient numbers
then all will be reasonably accurate or, for small population samples, at least reasonably likely to be conservative .. CG problems notwithstanding ..
170lb/77kg was based on a US military study from a LONG time ago and was still reasonably accurate in Oz when I was flying for AN .. on every occasion we choose to weigh the punters (F27, usually, when we were after every pound we could muster) the standard weight calculations came within a bull's roar of the scales figures.
CASA's John K did a very detailed study many years ago based on Oz medical data and came up with the basics which made their way into the CAAP. Provided one is sensible and pragmatic, I see little concern in using the CAAP data for a general Oz population sample .. albeit that we are getting a tad fatter as the years go by .. or is that just applicable to me ?
Silly to use standard weights for small aircraft ... but once you are up to a pax load of, say, 10-12, the standard deviation starts to motor down and you end up with sensibly accurate and commercial weights.
This presupposes that the population basis is appropriate and it is here that the system at the operator's level falls down. Unless the crews and loading folk have some idea of the population basis, then it becomes a bit tenuous.
A more sensible option, for a given operator, is to have several approved (or accepted) sets of standard weights to cover whatever passenger populations might be typical for that operation .. with the proviso that, if the passenger set patently is not representative of the statistics, then it is back to scales for a particular flight.
Quite a few years ago I flew a 757 to Athens ..
Amounts to a fraudulent misapplication of the system .. but, unfortunately, altogether too commonplace.