As a kid, I always had problems with the keyboard. Membrane was hard to come by and expensive. I did not have Kempston/Protec. So I had to play on the keyboard.
I was using QAOPMN combination, and shortly it would wear off. Then I had to try to fix it manually, by putting wire and what not. Usually did not work.
In any case, I always wondered why Sir Clive chose the membrane. I suspect it could be to cut down the production cost, but I am not sure.