In NL we've moved to a card system like that some years ago; the amount of people traveling without a ticket in the subways and trains has declined by a lot, and consequently, crime and violence against the train staff that fined them if they were checked. Likewise, staff could be reallocated from appearing as a big gang of ticket checkers at the exits of random stations to other tasks.
They do tend to put at least one guy near the gates though, for one to help out people that have trouble with the system, and on the other to discourage people jumping the gates. They're quite high so that's quite a feat, but there's also a builtin system so that it opens if pushed hard enough (in case of emergency).
Anyway. Ticket machines are big and expensive machines.