I do it because it takes longer. If everyone opted out, the lines would become so ridiculously long that they'd have to change their system. I'm doing my part.
I wish more people did it, though. Whenever I fly I'm the only one I see opting out. When I was flying from Munich to DCA they had one of these scanners installed to a lone US-only gate, and I said I wanted to opt out, and the (German) airport employee looked at me surprised and said "why?". He couldn't understand why someone would opt out of the machine, I guess he thought it was some kind of metal detector or something.
After they direct you to the scanner, just say "I can't lift my arms above my head" and they'll put you through the metal detector and swab your hands. They don't "test" you because if it's true, they could hurt you.
I've been doing it since last fall (30 flights?) without issue now.
(These have all been US domestic flights.)