I think that there appears to be a dearth of "good" centres these days partly because it isn't quite the specialist position that it was in times past. Nowadays you are as likely to see a centre also playing second row and vice versa, or a scrum half/stand off also playing hooker. Similarly a prop and second rower being interchangeable. In times of need clubs tend to stick anyone on the wing or at fullback. It would seem that the old specialist positions are now loosely broken down into just forwards and backs. Can you imagine an injured Harold Poynton being substituted by Geoff Oakes, or Neil Fox being replaced by Don Vines?? I think not!