My understanding is that the value in the birth plan is mainly in the creation of it, because it prompts women to be informed and to have an opinion on some of the choices that may come up during and after birth.
I'd imagine its also good to have things written down in black and white so that its clear that something is important to you and that you've thought it through so that it is paid attention to.
Of course we all want healthy babies and to be healthy ourselves. But I also have opinions on what I would prefer to happen if everything is going well (and indeed some ideas about what's important to me if things are going less well - like having my DH stay with our baby, get skin to skin contact as soon as possible and for the baby to be cup fed colostrum if we are seperated).
I'll totally understand if because of a medical emergency I don't have the birth that I'd like to. But it would be daft if everything was going well and things happened that I wasn't comfortable with, just because I hadn't made my preferences known. It's good to write them down because I don't know if I'll feel like explaining things between contractions to every medical professional who might appear.
In my maternity notes there is a section labelled 'Preferences for Birth' which has gentle prompts about things I might want to specify for labour, the third stage and bonding after birth. :) It has ideas like 'I'd like my birth partner and I to discover the sex of the baby ourselves rather than having it announced' etc.
I agree with the poster above who said her worst nightmare would be being forced to lie on her back, and be immobile. As I have anxiety issues, making me feel terrified and out of control during birth isn't likely to help matters.