I don't own a Birkin but do own two Kelly's and have now for 12 years. I tried a Twilly on one - I felt like a 12 year old girl. I don't think that poor Twilly as left the box since!
I don't use bag organizers, either. I am careful about what I put in my bags. My ballpoint pen has a leather case into which it slips - no pen marks in my bags. I keep it in one of the pockets along with two small CC-type organizers (non-Hermes). One, I use for all my health insurance cards - no need to get my wallet out to find them at the doctor's office. The other is even smaller, and I use it for dry cleaner/shoe repair receipts. I cannot stand having paper "trash" in my handbags! I even had a small leather envelope with a snap for sales receipts from stores. My cosmetics and other small things are in a leather zip bag, my regular eyeglasses are in a great leather case that Talbots briefly made several years ago - very Hermes-looking, My phone fits perfectly in the other pocket, and anything I want to always carry (sales receipt for Kelly - won't have to search for it if I'm going out of the country - lives in the zippered compartment along with my a small folder from La Maison du Chocolat with my DD's favorites circled.
Why would I need an "organizer"? It simply adds weight, and to me, takes something away from the lovely leather lining in an Hermes handbag.
You know how you live with your bags. If you carry water bottles/baby bottles/messy snacks etc, then a liner will certainly help protect the bag.