I've used Django Admin, and it is definitely really cool. However, I don't think it comes close to being a requirement for web development.

The awesome thing about Flask is you really only need to use what you need to use.

Flask also has really great support/docs/etc. I haven't had to google much to find answers to even the more obscure (use xyz supporting library) questions.

Not to be a Flask fanboy... Django definitely has its place. Flask works well for me.