Passport control and security are 2 entirely different things.
Personally, I'd not want to sit on a desk with the prospect of silly comments from half peed chavs or the paperwork from a PA claim made on dubious grounds.
ohjin22 - Yes, you probably did hear about the terrorists that got caught after security, or the ones that tried and failed after getting through, but how many were deterred that you don't know about? It's an unquantifiable. Just as the people I called idiots above, probably were, but who's to know if one of them was intending to leap out of his seat deactivated granade/real CS Gas/Knife in hand? They were all found by security. So is it effective really? Yes, it is. Nothings gone bang on a plane, and no ones hijacked one from the UK for a very long time.
If you want 100% security, then you're in the wrong place, it's not possible. Terrorists are not thick, they are inventive, the example of silly Irish Nurse is a good one, explosives in the lining of her bag, NO detonator attached, that was in a working calculator quite separate, perfectly viable device that was only found because an experienced screener wasn't happy about the weight of the bad when it had been emptied. But, even she didn't know what she'd found, nor did anyone else until an explosives officer arrived.
The Said Nurse, was completely unaware, although the logic of why her Jordanian Boyfriend was sending her to Israel, seemed to pass her by.
So that's why you infirm old lady gets searched, the DfT specify it must the the n th pax, and come hell or high water thats who gets searched. Just because she's disabled, doesn't means she can't be duped. Also, her chair can't go through the detector, which is another reason why it gets a hand search.