My ticket gets checked less then monthly and I commute to work daily. The monthly ticket costs at least 50€, the penalty is 40€. From a mathematical point of view, it makes no sense to buy a ticket.

There are a couple of caveats. When I don't have a ticket, e.g. because I forgot my wallet, I'm on a constant lookout and the train ride becomes really stressful. If you get caught several times they might press charges against you, so it is not feasible to go without ticket on a regular basis anyway. And last but not least, I'm using a service, so I pay for it.