If airlines started weighing passengers, I'd never fly again! And I wouldn't put it past them to start doing it either!
However, to help with the problem of being "overweight" I did see some nifty little scales the other day that can weigh bags up to about 25kg. They were small enough to take with you on holiday for the return flight. Certainly beats balancing a suitcase on a pair of bathroom scales and trying to work out how much it weighs! I think they were available from Amazon for about £20 so they should repay their price over the years if it means not paying excess baggage charges.
Our suitcases are heavy to start with - one weighed 6kg empty so I'm going to invest in a soft holdall...and resist bringing any wine home with me...:rolleyes