This is really fucking stupid. Here are a handful of reasons as to why this is a fucking waste of time and money (and yes, time is money); I'm sure you can come up with more:

1. If the scanner goes off, you can talk your way out of it. Damn, just when I thought that the three biq questions were tough enough, they show me that we can go way further.
2. We'll scan residue from people's hands, but we still won't bother scanning checked-in luggage.
3. The time wasted questioning all those false positives could be spent, I dunno, doing "security" things.

Don't worry everyone, we're really safe.