The biggest challenge that passenger rail faces is how to attract car drivers. Once a driver is in his car, it is infinitely more difficult to entice him out and onto a train. In particular, the costs of the car journey tend to be seen as invisible overheads (insurance, maintenance) or incidental (fuel), simply because this is something paid for routinely. This is why trains have things like catering and wifi, that cannot be provided (as well) in cars. Because car parking at a station will be seen as a consequence of travelling by train, I would suggest that the perception raised by excessive charges will be that the train, rather than the car, is more expensive. Result, the traveller starts his journey in a worse frame of mind, and feels less disposed to accept any failings in railway provision. This is why anyone with the interests of rail at heart should resist this sort of thing.
From the NCP point of view, it is probably quite poor business as well. The best business model is to find out what the customer wants, then provide it. A 30-minute free dropping off period may be a loss-leader, making drivers more inclined to accept higher charges at other times. However, from the apparent pushing of season tickets, it would suggest that NCP think they can fill the car park from regular travellers.
And does anyone know what a car park fee actually gets you. Certainly not any security!