I'm with bookworm on this. How often have you been given very precise traffic information when under an RIS, but totally failed to see the traffic -- even when you know where to look. Personally, I fail to see more than half the radar contacts called. See and avoid does not work well alone.
RIS works because you are given a 'heads-up,' you are looking out, and you will (almost always) see the contact if it's going to become a real collision hazard.
Most of us complain about compulsory Mode S for VFR in the future. However, compulsory squawking will greatly reduce the chance of collisions such as this Herfordshire one, either through better visibility to radar services or through better technology in the cockpit, such as the mini-TCAS systems which are now coming on the market.
AA.