You only have to visit London to see how many people jump the gates, I’ve seen many different people do it. And regardless if gates are unstaffed you have to have a way for people whose tickets are rejected by mistake to get out. Fare evaders can use that.People certainly do jump the gates but I’m not sure it follows that the benefits of gatelines to countering evasion are slim. They will deter all but the most hardcore of fare evaders due to social embarrassment/shame; not that many people are brazen though to push through gates.