Parish councils can contribute to the cost of local authority staff if it's for the betterment of the parish and proportionate. A Rights of Way Officer is not a statutory post but the local authority has a legal obligation to maintain and keep open PROWs so even if no parish council funded a dedicated officer, the job would still need to be done by someone. The long and short of it is, the local authority have to provide the service a Rights of Way Officer carries out in your parish whether you pay for it or not so whether there's value in contributing depends whether you think having a dedicated officer is for the betterment of your parish or if there are additional services they will offer if you chip in.