I think they can be positive if they are polite, sensible, and well trained in railway ticketing rules, for example split tickets, break of journey, and cases such as splitting tickets at a station the train doesn't call at using a season ticket or ranger, which are permitted. However, I was once wrongly accused of having an invalid ticket at Woolwich Arsenal, which senior experts have advised was a valid ticket. I have also encountered RPIs who lack regard for public safety. 2 years ago, I had a ticket inspection by Thameslink RPI on a train just as it was about to pull into a station I was due to alight at, and was at the door ready to disembark. The reckless timing could easily have caused a lost wallet. On balance, I am against RPI deployment therefore