Here is what is interesting.
#1 - actually I could argue that the old method of measuring is more accurate. Measuring serve speed as it crosses the net is more what the returner is actually facing. If Roddick hits a serve 140mph and it is 128mph as it crosses the net...the serve returner is NOT seeing a 140mph serve, he is seeing a 128mph serve (or thereabouts). Obviously it is more 'exciting' saying Roddick is serving 140mph...because he is....but what is the returning really facing? How fast is the ball going? Certainly a complex issue, measuring at the net is close to the midpoint between the server and the returner, and is a reasonable means of measurement.
#2 the old guns were highly accurate. If we want to say the measurement point is inaccurate, then fine. But to say the guns were inaccurate means they might be +/- 5 mph off, which they were not. The guns themselves were accurate.