I don't trust them to not save the images, I don't trust them to not get hacked, and I don't want some random black hat to have images of my body over time.
You'd just build up a body of images of "person 1" then cross-reference with flight records till you narrow it down to only being one possible person who had been on all those flights...
You've got head shape, hand shape, height, shoulder width, BMI, and length of every major body segment. From there you can calculate all sorts of ratios to apply to people, and I'd bet you could make a pretty good guess at gait for motion analysis.
A database like this would also be proof against facial surgery; changing bone lengths is harder. So if you get a known terrorist once, you should be able to spot them much more easily in the future. And of course you'd keep the records indefinitely, because you never know who's going to turn out to be a terrorist. Or a drug mule. Or a political protester [1].
[1] http://en.wikipedia.org/wiki/COINTELPRO for the historical version; http://nymag.com/news/features/nypd-demographics-unit-2013-9... for the modern one.
This technology could be plausibly misused for security purposes, and then perverted for political purposes, just like previous security technologies/bureaucracies have. That shouldn't make you upset at the people pointing it out. Instead, direct your outrage at the portions of government that have previously done exactly this sort of thing.
I think the powers of the state to spy on its citizens should be minimal and tightly controlled, because historically they always have been abused, and modern technology makes it much easier to run a surveillance state. So if you're asking how far surveillance should go, I'd say, "quite a ways back."
I'm not particularly worried about it, though, not in the US. As my links indicate, we've been through waves of this before. As long as government stays in the hands of the people, I believe this sort of thing will come out, cause outrage, and get pruned back.
My real worry is the rising Gini coefficient and the rise in intergenerational correlation in wealth and power. If we end up with something like a ruling class, then that cycle of error and correction will fail. Some would argue that the failure to punish anybody for the crimes leading up to the economic crash is a sign we're already there, but I don't believe it's true yet.