HD,
Is it really that significant a security risk?
Anything you could do from the roof of the QB, you could also surely do from the top floor of some of the car parks, or from the car park of The Renaissance, or from Myrtle Avenue. All of which would be much more difficult to secure than a formal viewing area.
I suspect that the lack of such a facility is more to do with the headaches BAA don't have as a result of its absence...corporate management tends to have little in the way of sentiment or soul these days. A public viewing area just doesn't generate the income!
Saying that, I wonder what it would take to open up the old control tower to the paying public. No better view, and could be secured fairly easily...I would certainly pay a tenner for an hour or two there...