http://www.bbc.co.uk/news/business-41835933
No doubt, if they decided to use a price per kilo for a ticket we would not gain from it !!!
In truth, probably inspired by IATA to see if the present day `notional` weights of passengers is still valid. These `notional` weights are used by airlines to help determine the gross weight of any aircraft prior to departure. From such weights they work out fuel requirements and speeds for take off.
Makes sense to recheck the figures from time to time