I think it is a good move. It speeds entry (and therefore eases the traffic flow), prevents the ridiculous overcrowding experienced at some Duxford shows where the gates have to be closed and the surrounding roads are still clogged with people trying to get in, and in general allows the organisers to control the size of the crowd. It also removes the risks to the organisers associated with weather. It is just the same arrangement as many large events (concerts, sporting events, etc) have operated on for years.