That seems to be a complete waste of time and money, something for people who think they are being clever and smug getting one over on the airline/staff.
Anyway, if someone was to wear one of these things and carry the same amount as they would do in an average hold bag then it would be a bit bulky. The kind of person who would wear this would no doubt have a carry on bag at maximum dimensions too (as is demonstrated on the website with that rather arrogant man in the video who thinks the 1 bag rule doesn't apply to him*) so they'd be taking up more space, which is unfair on other passengers who managed to pack sensibly. If they insisted on wearing it (it is a coat after all) then it would overflow into the neighbouring passengers space, which if it were me, would not be well received!
Also if they were sat at an emergency exit I'd suggest to the crew that perhaps that bulky coat may impede their ability to operate the doors.
*He still needs to hide the jaktogo from the second member of staff, so not exactly beating anything, just cheating. Also notice it doesn't show him actually on board, I wonder if the crew took it off him in the end?