Each to their own. I wouldn't be able to pass up the 19w u/s, but I think its fine if people make their own decisions based on their personal beliefs.
A good midwife should be able to pick up twins from feeling/stethascope or the like, so I can't imagine that being an issue.
But I do understand where you are coming from. Some people have to do everything in their power to get pregnant, and others can seem to ignore the medical world and everything still goes OK. Thus us the human race I guess?