It's a case of causing inconvenience to the majority of honest, fare-paying customers for a minority of people. Firstly, the gates at Nottingham are not manned by RPOs as standard, usually CSOs, and there is limits to what they can do, they certainly aren't authorised Penalty Fare collectors and they aren't PACE trained to interview under caution. I have observed at Derby them just letting people through without a ticket and telling them to buy one from the ticket office - which I doubt a hardened fare evader would deign to do. It will also make the NET tram stop inconvenient for station access and make changing trains when requiring to cross to a different platform a hassle. Ticket gates don't stop doughnutting (buying a small fare ticket to enter and leave the station without paying the correct fare for the whole journey).
The set up at Sheffield works well, with RPOs being on the vast majority of services - Rail Safety Accreditation Scheme qualified officers who can involve themselves in violent, high conflict situations and equipped with stab proof and bullet proof vests as well as personal CCTV. It's rare to see a PF or MG11 being undertaken on this route, as people know they will likely be caught and face consequences if they evade the fare. With the introduction of virtual ticketing, it's quite common to board a local service and see people playing dumb by having an invalid ticket, usually an advance that's not valid - ticket gates at stations won't stop people doing this. Stations such as Nottingham and Derby have "Travel Safe Officers" who do seem to look rather bored most of the time, so perhaps they'd be better off on the trains supporting revenue staff/Conductors/Train Managers and working to prevent assaults on staff instead of loitering around major stations which already have a BTP presence.
As a customer of the railway, I have a right to an opinion and mine is that ticket gates are a nuisance to me and I'd rather they weren't there. I pay my money in to the network so am entitled to an opinion on the matter. Stepping up RPO patrols on board the trains is the way I'd rather they went to protect revenue. Services that are rife for anti-social behaviour, such as the Skegness line could really benefit from having Officers checking every ticket after every single station. This would be a pro-active way of protecting revenue and preventing short faring without causing an unpleasant experience for their genuine customers.