I've been wondering about this too. I'm still fairly young (27), healthy, and we skipped all ultrasounds (except for Doppler use once early in pregnancy and again in labor) with my first pregnancy. I'm nervous about starting a cascade of interventions if we get a positive screen, because we'd almost certainly go to an amnio at that point. I just don't know what we'd do with the information. I don't even want to hazard a guess because I don't think we could really know unless we were in that position.
I also may be one of very few pregnant women on the planet who is concerned about the safety of routine ultrasounds in pregnancy. I doubt that one or two is a problem, but unless there's a recognizable reason to do one, I try to avoid them. But of course I would like confirmation that the baby "looks" ok; who wouldn't? I have to think about it some more.