Ian, I'll email you directly with some comments on this.
I think it is a very good idea, personally, although unfortunately, it's applied inconsistently. All enthusiasts - and indeed, anyone using an airport - should be on guard/alert at all times for suspicious activity. The problem is that whether this approach is taken varies very much from airport to airport. Gatwick, for example is very anti-enthusiast (Heathrow, not much better) and provides absolutely no facilities; the point is that if an airport provides a vantage point where all wishing to watch acft can congregate, it cuts down on the number of people seeking various different nooks and crannies around the airport, which in turn contributes to suspicion.
Naturally, authorities are keen to be seen taking a tough line in the event of a serious terrorist threat, but it can be far more effective to engage the Mk 1 brain first and make sure that all possible assets (including enthusiasts) that can be used to provide info and prevent an attack are foiled. Making life as difficult as possible for enthusiasts, while also expecting them to co-operate doesn't seem like particularly good psychology; it's all take and no give.