If those brief, minimal x-ray scans we get during TSA searches is worth considering, then I wish someone would do something about shielding us while we're flying, and getting a great deal more radiation...
We take risks every day and that's completely okay as long as the benefits make up for it. I'll take a 0.1% chance of dying for a million dollars, but I won't take it for a sandwich; I'll take a bit of radiation to fly, but not because of a half-assed security mechanism.
It's a classic false dichotomy. Specifically, a person can care about both issues simultaniously.
The problem is that we're not producing the radiation in the air -- it's already there. Radiation shielding is heavy and infeasible for use in current airplane technology. It's a very, very hard problem.
BackScatter scanners, in contrast, are optional. They are completely and utterly unnecessary. It's very easy to get rid of them; indeed it would have been easiest and cheapest to never have forced them upon an an unwilling community of travelers.
But you get _nothing_ back by going through the TSA scanners except getting your cells blasted by energy enough to mutate your DNA. Yes you get the same in flight, but you are getting some place pretty darn fast as a trade-off.
To continue the analogy. Imagine you go through a machine every morning that has a spring loaded hammer when you exit you house to leave. Most of these machine will do nothing to you, but every rarely those machine are known to release their hammer and smash limbs or cause injury otherwise. That is exactly the equivalent of TSA scanners. You get nothing back for the risk of injury.
What I was trying to (and failed to) do was focus on the fact that I'm getting radiated while flying - and I don't ever recall that being talked about in much detail, and certainly not by any airline staff. For example - do I get more radiation from an open window? Do they make window screens that can block radiation? Is there any research into reducing radiation exposure while flying? In much the same way we take a risk while driving, but now have Air Bags, ABS Breaks, tempered glass, break away wheel columns, etc... - I'm wondering if we're putting the same effort into the radiation risk while flying.
I just never heard much about it until the TSA added the backscatter machines.
Since you are part of the traveling public, you can further simplify your sentence to: "my opting out poses less risk to the travelling public than being scanned". That's like a 5-word trim! :)
Or to flip it around, I opt out because I happen to know something that the TSA doesn't: I pose no risk to the aircraft, and scanning me in no way makes the aircraft safer. I can therefore assert with absolute certainty that my opting out poses less risk to the travelling public than being scanned poses a risk to me. (Plus, I think I might secretly enjoy TSA agents awkwardly and apologetically rubbing the backs of their hands on my crotch.)