Ermmmmm - because it is not a legal way of navigating, perhaps? Or because it causes drastic loss of situational awareness, and the CAA at least blame this attitude for the recent increases in airspace infringements?
I agree that correcting heading by the track error given by GPS and using the ETA function is better than MDR for that leg. However what if the GPS does not have a signal for the next leg? At least the person you refer to would have a wind more accurate than the forecast, if a little out of date. He also keeps in pracdtice with his MDR for use when it is really needed.