I can say that I was required to use one to get back into the U.S. while transiting through Schiphol back in October 2013. No opt out, just the scanner and "I'm not forcing you to fly today - you can stay here." Pretty rude, too.

Since then, I always transit through Heathrow. It's a logistical nightmare with all those buses, but they've never coerced me to go through a scanner. Those biometric checkpoints, on the other hand...

[Note: This was with Dutch staff for British Airways; not sure if that's relevant.]