Personally, I decline the scans because it's my one way of protesting the security theater and its enormous waste. It's the one thing I'm allowed to decline, so I decline it.
Here's a really good way to protest security theater (albeit very much at your expense): write SSSS [1] on your boarding pass, and see how you're treated! (Security officers can write SSSS on your boarding pass, so it doesn't need to be printed on the pass, so this will work...)