IIt is true to say that the reliance is placed primarily on the u/s result providing it is done by a highly skilled and experienced operator. But I don't believe it is true to say that it is not helpful at all in this screening process. I do believe it is the cause of the high number of "false positive" screens and I have also personally witnessed this also. So, that in itself is an issue.
Just thought I'd add this in case anybody gets confused ... false positives are guaranteed at being close to a certain rate for screening tests like this. For example, they will pick a ~5% false positive rate for a ~90% detection rate. Depending on the exact method used, thats about the rates you get for NF+bloods. They could reduce the false positive to say 2% but the detection rate will drop to say 65% (last figure a guess just to illustrate what I'm saying). A 5% false positive means 5% of *all* women screened will get a false positive - so 1 in 20 will get a positive screening result even though most of these will be fine. (It doesn't mean 95% or even 5% of positive screening results will have a positive CVS/amnio result - its down around 1% somewhere).