I'm not sure there is one right answer or one perfect standard for every event. Certainly at nationals-level events the sailors are able to handle a wider range of conditions (wind and wave) and will tollerate fewer mistakes from the RC before crying foul. However, at local events with A, B and C class sailors all toeing the line together, it's often as much about about the experience and fun of racing as scoring a high placing. In these "local" events, to pull the plug on a race because the wind dips below 5 knots seems a little too strict for the spirit of the day. At a national-level event, by all means, lower wind limits are worthwhile for maintaining a fair play, as are upper limits. I think it comes down to the spirit of the event and having the judgement to handle the situation fairly.