I think you only need your valid ticket to not work a few times before the average member of the public starts to wonder if they are quicker going through a manned barrier. A similarity may be the self service tills at supermarkets. On the whole they are probably slightly quicker but people just get a few examples in their head of when it isn't and its game over. I have a valid Trio ticket in Merseyside at the minute so it is a none starter and I have to use a manned barrier. I'd be quite happy with a smart card if I knew it would work.