I think they're really important for lots of reasons. Even if you don't take it with you the process of doing it is valuable in itself.

1. The process of writing it forces you to think about the birth and prioritise what's important to you
2. The process of writing it initiates conversation with your partner about what you want and how he can help you in labour ("hey honey, I was working on my birthplan today and I was thinking...such and such" - it gets you talking which is always a good thing.)
3. If you're going private and seeing the same OB who will be at the birth you can show it to them beforehand - it's a fantastic way to suss out your OB's true feelings about what you want BEFORE you're in labour and in a position where it's hard to negotiate.
4. Because it is a written document it must be put on your file as part of the hospital record keeping protocol - making it more likely that it will be taken seriously.
5. It gives you input and a measure of control that is taken from you in hospital if they can see you will just submit to protocols and procedure without question.

One thing I would say about them though is to make sure you understand what you're asking for and why. As an example, say you're going in for an induction with the drip but you have on your birthplan "I want a natural 3rd stage of labour". These two things aren't compatible and it says to the hospital staff that you haven't done your research properly and you don't really know what you want. I think this is a big reason alot of birthplans fall over, people don't research their options enough.