Hmmm. I think of it this way. My mom is a hairdresser (give me a chance, I think this'll make sense). She's a really, really good one. She's precise, she is great at customer service, she stays up on all the latest technology and styles. She uses an assistant to do things like shampoo, take down perm rods, and clean up her station between clients. These are things that first, allow her to be more efficient. She can get more done with more clients if she has her assistant to help her. Second, it is physically easier on her, as the assistant does some of the work that hurts her back, and none of the work that will affect the outcome of the client's hairdo. Third, my mom is able to spend more time (sounds strange, doesn't it?) meeting the EMOTIONAL needs of her client if her assistant is doing some of the, uh, well, some of the grunt work.
I feel the same way about an apprentice midwife. As an assistant to a hair dresser is often working on their own license and is there to watch and learn as much as to make a buck, a midwife's apprentice is there to watch and learn as much as she is to do some of the grunt work. When some of the little, mindless details that the midwife wouldn't want to give to someone NOT trained in birth are covered by the apprentice, the midwife will have more time and energy to put into supporting you in the manner you need.
That said, I know of several people who didn't click with the apprentice and asked them not to be present at their birth. No problems, either. Just know that, in all, your experience might be a BETTER one for the extra, quiet, efficient body in your home.