@MissDaphne - They probably just didn't bother mentioning your grade to you. They would definitely have checked the placenta.
You are right that routine ultrasound, though common and usually covered by insurance, is completely optional. But there might be cases where ultrasound would be medically indicated, like if the mom was experiencing unusual pain or bleeding. Sounds like you experienced that yourself, with the subchorionic hemorrhage. That must have been scary!
I have a friend who is an ultrasound technician and she was telling me how they check for placental calcification at the end of pregnancy. Apparently, doctors sometimes get worried if a mom goes "overdue," even though technically anything up to 42 weeks is "normal." Checking the placenta and other aspects of baby's environment via ultrasound is one way that they make sure baby is OK to stay in a little longer vs. needing labor to be induced.