A lot of doctors say they give women unrealistic expectations and are dangerous. Some doctors are openly derisive.
Many midwives also scoff at them.
A lot of women say "you can't plan your labour" and decide against one.
The birth plan is not about dictating that you will have morphine at 2cm and then an epidural at 4cm, nor is it about getting into a bath at exactly 5 minute contractions. Labour doesn't work that way. They also aren't about candles and essential oils and a willow tree (although some women do choose these things!).
A birth plan is more to do with seeking information and deciding what options are best for you and your baby. I encourage women to add things like whether you want to be offered pain relief or ask for it yourself, whether you want baby skin to skin after birth, and whether you want your third stage to be actively managed or physiological. Put in your feeding preference (if you have one) so you aren't asked fifteen thousand times if you're planning to breastfeed or formula feed. Note down your preferred support people, and any cultural requests. The type of things that you CAN control. And you can also investigate alternative options if things go awry, when you might not be in the right headspace for rational thought. They can also help your partner to advocate for you. Even women who are having a planned caesarean can benefit from a birth plan!
Birth plans are about YOUR CHOICES and what is important to YOU. Going into birth armed with evidence-based knowledge and clear lines of communication empowers women. In today's medicalised system, women often feel as though they aren't being heard. A good birth plan articulates what's important to each woman in a way that she in comfortable with.
And if you choose not to have a birth plan, that's fine too! We're all different.