I would not have DC2 had I not had my anatomy scan like missmedusa. Incompetent cervix is only discoverable by ultrasound. I had nearly 20 scans with her following the life-saving surgery to check on things and so on. She's a little bit bonkers at five years old but I am pretty sure it has nothing to with the scans and everything to do with who she is 
Currently one of my friends is on bedrest in hospital with placenta accreta. If she had gone into labour and was unaware of it, she could well have died along with her baby. Thanks ultrasound!!
I have another real life friend whose IUGR was only discovered thanks to a growth scan at 28 weeks - she may not have her DS if it hadn't been spotted.
There are so so many dozens more examples I could give you too..
It's surprisingly common to find something on ultrasound that is threatening to the life of the baby (and sometimes mother) if it's not known about. It's not just about abnormalities with the baby and deciding whether or not to terminate as the author of the link suggests!
I remember reading all these things a while back and being concerned, but ultimately the benefits outweigh the not entirely proven risks so many hundreds of times over that it's a no-brainer really. Massive benefits.
Although I disagree with people (uh, Tom Cruise) who buy their own machine and check every day as there's not enough research into safety at those levels. But funnily enough most of us don't do that!