I think that birthplans are just that, i.e. plans and we all know what they say about the best laid plans...
For my first I thought it was compulsory to write one, as I thought it was to go with minimal pain relieving drugs, favourite music, hot/cold flannels, lipsave, quilt, extra pillows, glucose tablets, birthing stool (couldn't actually find one), good book, short (but not too short) nightie, snacks for husband, TENS machine, lavender oil (organic obviously), etc. etc. Without going into details of 25 h labour none of the above was of any use as I found my body took over and fortunately my brain (and the very kind words of a super-supportive GP) was still fully engaged and I was able to make the right choices for me at that time.
Birth no. 2 (again with a plan albeit a few lines) was in a wonderful maternity unit where I was fortunate to have established great relationships with the midwives. They knew me well enough to ask before doing anything I might have objected to but this didn't arise. Fantastic birth - very relaxed and confident.
Birth no. 3 with no plan in an unfamiliar and not particularly friendly hospital was still superb and I think this was because I knew exactly what to expect and what I can tolerate. Good job really as first contraction at 2.30 a.m., waters broke immediately after and son was born with 2 puffs of entonox at 4.05 a.m (boy were the roundabouts en route to hospital painful).
In short, I think birthplans are great the first time as you feel you are doing the right thing but the next time you learn to trust your own instincts and are able to convey these from experience (you are far less likely to feel patronised when you've been through one labour).
If there was ever to be a next time I don't think I'd even consider one - would I have the time?