I had a prenatal nurse and it was great. I could call her anytime I had questions or concerns, which you can't always do with a doctor.
She was able to reassure an older, first-time mom with the jitters that I was going to be OK, that my baby was all right, and that everything was going just like it should. I was so grateful for her care and my daughter was born without complications and was absolutely perfect and healthy.
In my opinion, a pregnant woman needs someone she can call with concerns, outside of office hours. She will benefit and so will her baby.