I don't think birth plans are a waste of time, but I prefer the more modern description of 'birth preferences'. The point is to think about what you want in all situations - what if you need a caesarian? What if you need to be induced? What if you want a birth centre birth but the birth centre is full? Ideally, what would you like to happen in those situations?
I agree you have to be flexible - for instance in my case I wanted a water birth in the birth centre but my first baby arrived too quickly and I had to give birth in the delivery suite instead. No water birth.
However, I'd made a playlist which was played for me. As part of my birth preferences I'd requested a female obstetrician to attend me if an obstetrician was necessary and it was possible - this request was honoured, as I did require some interventions (my waters needed to be broken for me and I needed an episiotomy).
Other things were not respected so much - eg I'd wanted to give birth in an upright position but I was told to lie on the bed and give birth on my back, despite the delivery suite having posters up showing alternative optimal positions of birth.
I found that preparing my birth preferences and using the techniques of hypnobirthing to be useful, as they made me think about all different scenarios of labour and what I would like in each circumstance.
Friends I know who have gone in blind without having thought about anything found the whole thing more traumatic than I did.
I dislike the way women are sneered at and belittled for wanting to have a 'good' birth that reflects their wishes. There are plenty of births out there that do go to plan - look at all the positive birth stories on YouTube. Lots don't, of course, but the idea that all women end up screaming uncontrollably for pain relief or being rushed into surgery is false.
I also suspect that obstetricians are more cynical about birth plans/preferences because they only see the cases where their intervention is needed. Midwives, who see a broader range of straightforward births at home and in birth centres, who are present where births do go smoothly and in line with the mother's wishes probably recognise the benefits of drawing up a set of preferences.
I think the problems come when one is too attached to a particular outcome and is unable to adapt when the birth doesn't adhere to the ideal.