That is just bizarre. I'd say it's her that finds them 'insulting' and that speaks way more about her and her issues than it does the validity of making a birth plan.
For all the reasons you've mentioned, it's a perfectly reasonable thing to do. You need to know that your wishes are clearly communicated to whoever is there and it allows you and DH to think through all the possible contingencies. I think my only reservation about birth 'plans' is if they become birth 'expectations'. If you hold on too tightly to how you think your birth will go, that can certainly trip you up. But having your wishes clearly stated... to me, that's just good sense!
Definitely do one. As PP say, it's good to get your wishes down on paper. I think it's presumptuous of the midwife to assume that your birth plan would only include 'routine' things - there are many preferences when labouring and giving birth that can differ from person to person. And it can be hard to remember, let alone communicate these, in the throes of labour. I know DH found it useful to refer to. Assuming that all her 'patients' want exactly the same thing is like going clothes shopping and all the clothes are the same!
I kind of get the standard routine thing, I hadn't written a birth plan before I did the classes but had an idea of things I was going to request, but after the classes realised whole chunks of what I would have written were just standard practice and didn't need to be requested specifically. BUT I don't think that would equate to not writing one, I think it is still really useful, and I think the idea of it being offensive is a bit.....odd. I found that what I did write was much more concise and easily digested once the standard stuff was out the way. I would suggest meeting with a midwife and asking some very specific questions about what is standard (i.e what is the normal practice when x or y happens?) so your plan can focus on what is different or absent in their normal methods.
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