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!