I think it's a good idea to have one, if nothing else, because it causes YOU to think about what you want and look into the options & alternatives. And then it's up to you to decide if you want to alter any of it when the day comes (like going for an epidural or not).
For me, I had a rough birth plan and I don't think the nurses cared to listen/look at it. And I know at least one thing I didn't want--an episiotomy--happened without my consent or anything, but oh well, it happened and it was because my doc thought I'd tear. (DD was 8 lb 2 oz so yeah I probably would have.)
I think a big thing is looking into what the SOP's are for your doc and for the hospital, because a lot of things I wanted were already standard for them both (nursing right away, skin to skin, etc.) so I didn't need to spell it out, they did it automatically. But that can be part of making the birth plan--finding out what they normally do and working with that.