I do agree with the OP that u/s can be notoriously wrong for Down Syndrome. But it can lead the parents to further testing, which is their right.
I'm not saying every woman should have 20 u/s willy nilly. What I'm arguing is that for the majority of women that have repeat scans, that it's for a reason. And trying to make these women feel guilty by the OP minimising their reasons in the name of a perceived safety is disingenuous.