well in my opinion I do not think they should be allowed at allfor my part they are very dangerous esp when trying to tie into and existing grounded conductor and you find out it is a neutral,and yes I agree with you about the warning.
On another forum in 1 week there was 2 different people asking about the outlets not working on bottom but was fine on the top and another had replaced some outlets and had problems
both casses were multi wire circuits that were not installed with the breakers tied together and the other guy thought they were 120 oulets and had no idea to cut the brass link
