I recently gave birth at ABC and my doula was super-helpful. Originally, since this was our first child, we were looking for just anybody who could come help us at the hospital, so that if my husband needed a break I wouldn't be alone. After we heard a story from another couple about how they were transferred to L&D from the ABC and their doula was a godsend, we ended up hiring a very experienced doula who had attended 60+ births.
We were very lucky to have her, because we also ended up getting transferred to L&D after 4+ hours of pushing with little progress, and for an hour and a half or so it was mainly the three of us (myself, my husband, and our doula) in the room. It was fantastic to have this constantly present person who knew what I was going through (my husband was really helpful, but he couldn't understand what I was going through at the end, and it was the doula who helped push me to push my baby out).
The nurses and midwives I encountered there were fantastic, but though they came with me to L&D, they couldn't be there the whole time, and the doula was my advocate.
Of course, since this was my first (and only) child, I can't speak to how it is at the ABC without a doula.