Firstly, people have been doing this for years - but on a small scale and very few per plane load. I've used overcoats and cargo pants on occasions.
Secondly, these folks think they are clever but it's short term and they will not be making a lot of money in five years time. Or even next year.
Thirdly, TT is right that the hand bagge problem will get worse, because people are going to want to store these in the bins. If they load a 5Kg bag and a 5Kg jacket - then the CC will soon start to notice and feed back to the carrier mgmt.
Forthly, as TT says, the weight IS a problem. Margins (of everything) are getting tighter and carriers naturally aim for 100% occupancy with less margin of error. If FC start assuming an extra half ton (or so) for safety and uprate the power settings for departure - are mgmt going to tweak their ear?
Lastly, the problem is not going to go away. Even in the good times, everyone wants something for nothing and in the deep recession - the more so. The problem is also of the carriers own making. They could have addressed this 10/15 years ago but chose not to because they thought it would frighten away the punters. Now they are trying to fix and it's going to be VERY difficult to undo all that learned behaviour.
Expect more temper tantrums to happen in the queue in front of you and in the seat next to you. Unfortunately, it may be you that has to do the protesting to the desk agent/CC about the problem.