The situation on the Robin Hood line is that the TVMs at many of the stations are regularly OOU and for a prolonged period. The one at Hucknall has been continuously unusable for at least the last 8 weeks. I'm sure that EMT are aware, so the problem isn't due to a lack of realtime monitoring, but just because RPIs are aware that they shouln't be issuing PFs, it doesn't mean there's no cost to either the TOC or customer.
Given the frequency of stops and with a full/standing train at busy times, the guard will have his work cut out to sell tickets to all those who have been unable to purchase beforehand. This either costs the customers (in time spent) purchasing a ticket (e.g. at Nottingham) or it costs the TOC (in lost revenue) There is also the inconvenience to customers of not being able to collect tickets bought online from a non-working TVM.
If TOCs are pro-active enough to rectify the faulty TVMs they are aware of in a timely manner, there might well be an argument for having realtime monitoring. Until that's the case, there's not much point.