When your MW sees what a wonderful asset your doula is to the support you will have, she will likely change her opinion. I think often times that is the concern midwives have. They imagine the doula will take over, try to persuade the family to dismiss the midwife's suggestions, generally get in the way and potentially cause a problem for mother and/or baby.
Many don't truly know the role of a doula simply because they have not experienced one at a birth. I once had a mother ask her OB if she could have a doula at her birth and he said yes, she could have her son there.... He didn't know what a doula was and thought she was asking if she could have her son called "Adoula" with her!
A doula's role is to make sure the mother and partner know what to expect, ways to be more comfortable, how they might help the labour progress, etc.. They offer suggestions for comfort to both the mother and partner....supporting both of them physically and emotionally. They make sure the family understands what is being discussed, what their options might be, what questions to ask. They are the advocate for the family...even when the family may be wanting to birth in a way the doula may not "agree" with, the doula's role is to support that family and their goals for birth whenever possible.
If they don't agree with a suggestion from the midwife, or if it goes against what the family was hoping for, the doula will make sure the family knows about potential risks, benefits, why it is being suggested, and possible alternatives to try first.
There is an art to suggesting alternatives or asking about alternatives that can be tried first without appearing to be second guessing the midwife. The doula will also let the family know when an intervention being suggested truly is in their best interest.
Unless there is a true medical situation that would prohibit something, and in the vast majority of births there isn't....everything is an option.
Most MWs find having a doula present actually frees them up and makes their job easier. Some doulas will have clients referred to them by midwives who have worked with them before.