Twillies are optional, but I love organizers! They help keep the shape of the bag and make it easier to find things. I have an organizer in each of my Birkins, even the B25s.
I am the same way, twilly is optional depending on my outfits and mood, most times I would go without the twilly.
However an organizer is a must, I have one for each bag because I carry only few things so I love how a well made organizer helps to keep my “stuff” organized in each individual side pockets without “swinging around” inside the bag. I don’t use SLG because that means i have to unzip the tiny bags to get to my things (probably need to look for it inside that tiny bag). I also love how a well made organizer keeps all my Birkins in shape, personally I don’t like the slouchy look of a Birkin unless it’s bigger size like a B35.