The "old wives tale" that you refer to is something that doctors tell patients about when predicting gender. For some odd reason, it is particularly accurate.
It is probably nice for people to know if their child has a congenital defect, but it usually isn't necessary because there's really nothing to be done about it. Furthermore, plenty of congenital defects are not detected by ultrasound and those parents adjust to the news just fine. If a baby is in distress at birth, the apgar score tells the story.
To subject all fetuses to a test so that parents of 2% of the babies can be psychologically prepared for an abnormality seems like serious overkill ... usually statistics gravitate towards the normal, not the rare occassion.
How many of these reasons for ultrasound are really necessary:
Confirm the pregnancy and its location. Some embryos develop in the fallopian tube instead of in the uterus. An ultrasound exam can help your health care provider detect and treat a tubal (ectopic) pregnancy before it endangers your health.
Determine your baby's gestational age. Knowing the baby's age can help your health care provider more accurately determine your due date and track various milestones throughout your pregnancy.
Confirm the number of babies. If your health care provider suspects a multiple pregnancy, an ultrasound may be done to make sure.
Evaluate your baby's growth. Your health care provider can use ultrasound to make sure your baby is growing at a normal rate. Ultrasound can be used to monitor your baby's movement, breathing and heart rate as well.
Study the placenta. The placenta provides your baby with vital nutrients and oxygen-rich blood. Any problems with the placenta need special attention.
Identify possible fetal abnormalities. An ultrasound can help your health care provider detect many congenital abnormalities. An early diagnosis may lead to early interventions that help save or improve a baby's life.
Investigate bleeding and other worrisome signs or symptoms. If you're bleeding or having other complications, an ultrasound may help your health care provider find out what's going on.
Perform other prenatal tests. Your health care provider may use ultrasound to guide needle placement during certain prenatal tests, such as checking a sample of amniotic fluid for specific genetic problems (amniocentesis) or testing a sample of the placenta for genetic abnormalities (chorionic villus sampling).
http://www.mayoclinic.com/health/fetal-ultrasound/PR00054
I changed some to green, meaning they are not good reasons for an ultrasound, or that knowing the information will make no difference. For example, if there is something wrong with the placenta the only thing you can learn is that there may be a miscarriage. Everything in green can be checked without ultrasound.
I changed some to blue, because those are reasons where some factor during the pregnancy gave a reason for checking whether everything is okay.
The one in black clearly says that
may help, but most often it doesn't change anything.