At my antenatal class we were told that part of the point of having a birth plan is so that your birth partner (OH) knows what you want and can act as your advocate. So when it all gets a bit much and you're screaming for painkillers he can intervene and double check with you that you really do want the painkillers and have given up on your plan, or can possibly suggest alternatives that you've discussed beforehand.
I love my OH dearly, but I know that unless it's all written down he will completely forget everything we've discussed beforehand. Actually he'll probably forget to look at the plan, or forget that we even had a plan!! But at least I'll know I did my best ;o)
I do agree Beanie, I want to set out a clear plan of what I want, but if it doesn't work out that way - so be it. The main thing is to end up with a healthy baby and not too many (physical / emotional) scars from the experience.