I think a reasonable snagging list before completion is as you say, much easier and cheaper than once a customer occupies. The balance of power is unlikely to change to the customer as there would be customers who would take it too far and demand work was done even if it was within the the standards.
Customers could delay completion and string the builders along indefinitely. Not saying the system works either way.
Most builders I know will do a home demonstration before completion and if the customer picks up any snags these are usually attended to.
It is not practical for new home buyers to thoroughly check their home during the new home demonstration.
Whilst the site manager will be happy to correct any issues spotted during a buyer's walk through, it cannot be considered an opportunity for an inspection. Indeed, many demonstrations take place often a week before legal completion with much work still outstanding even to finish the home.
As for buyers "demanding work to be done which is within standards" deemed by whom the housebuilders? The housebuilder's tolerances "get out of jail free card" for poor quality via the NHBC "Consistent Approach to finishes"?
If a buyer thinks it is wrong and not acceptable it is highly likely they are justified.
Finally if plc housebuilders knew in advance, that buyers would only legally complete when the home is finished to their satisfaction, quality would be improved dramatically and construction programmes extended to ensure better quality.
OK I admit there would be one buyer per development, that goes way too far as they say in the trade too "picky" but even they could be appeased with good site management and the right attitude.