TMA - total mass allowance

Recently heard baggage fees are going up again: http://www.airfarewatchdog.com/blog/3801089/airline-baggage-fees-chart-updated/, and also came across a friend's blog on "Unbundling of Baggage Fees": http://unbundling.wordpress.com/

This reminds me of my experience and conversations on Ryanair's everything charged a-la-cart model. I like my stuff, and carry my essentials everywhere I go. If we buy plane seat by the number, wouldn't it make sense for each seat to have allotted weight... not all bags are packed equal, some little carry-on's weigh a ton. Wouldn't be easier for the airline to manage load/balance by having everyone checking in stuff to step on the scale with their stuff (under x kg/lb is included with the seat purchase, and extra charge after that)?