The A-Head system has been a great invention for the BMX bike. The goosenecks from the early days followed by the Tuf- and Pro Neck type of stems had a few flaws.
First of all, with a little force you could move the stem in the forks, especially if you ran it high. The biggest problem was the headset getting loose all the time. When you were out for a BMX session, you always saw people grab the headset and turn the big nut and bigger ring to get the forks tight. Keep in mind that we did not have the right tools to counter lock the big nut and the ring. But even with the right tools, the headsets seem to get loose from time to time.
This changed with the invention of the Aheadset. I believe it was used on mountain bikes first and then got adapted on BMX bikes. Standard Byke Co were one of the first ones to change to this
system in the early '90-s. Rob-O had to take the heat for the MTB item on his BMX, but looking back he was helping all of us because this was the end to loose headsets.
Now check this photo from the 1980 May issue of BMX Plus! It shows a similar system designed by Cook Brothers. Can't believe it didn't become standard issue back then.