they are not for everyone, that much is true.
i think mars and others have ably demonstrated the point
i did a talk recently at a conference, not even about doulaing, and i have since found out that the consultant obs who was also giving a talk has started recommending doulas on the strength of my talk!
i have been told it was like having a mum/older sister with you.. without the baggage!
also, i have very strong links with the local hospital, and good relationships with HoM and Consultant MW which is useful
doulas don;t just do the birth, but hte run up to it, debrief previous births, help with breastfeeding if neccesary, have access to huge amount of info for mothers, and are a familiar and kind face for the parents in labour.
i have seen mothers and fathers visbily 'let go' when i arrive , and really crack on with labour.
i am not under any illusions that i am a replacement for a midwife or any other support, but another facet , another element of support... one that is unconditional, non judgemental and constant.
doulas have to keep themsleves updated on birth and breastfeeding info, and have enough knowledge to know when it is necessary to step away from the birth plan and empower and support the mother to do what she needs to