I think an advantage of the 20wk scan is that they can pick up any problems with the heart or whatever, so they can be prepared at the birth to deal with it straight away. It's not just to find out whether your baby is normal or not for those who 'might' terminate.. altho I can't imagine terminating at 20 weeks!
A friend of ours knew that her baby would need heart surgery straight after birth because it was picked up in ultrasounds. It just reduces the delay/risk of diagnosing after the birth.
That being said, they're not terribly accurate either, but I think they do serve their purpose. I do think people go a bit too far booking 3d/4d scans just for the sake of looking at the baby.

