Choosing a minimalistic web framework like Flask or Sinatra makes sense if you're experienced enough to know what choices Django and Rails make for you and you're comfortable making those decisions on your own. Otherwise, pick Django or Rails and start building things.
Couldn't agree with you more. I know what choices django makes for me, and I'm entirely happy letting it. I'd really miss a lot of what django has to offer if I were to move to something like flask.
http://www.python.org/dev/peps/pep-0020/
Choosing a minimalistic web framework like Flask or Sinatra makes sense if you're experienced enough to know what choices Django and Rails make for you and you're comfortable making those decisions on your own. Otherwise, pick Django or Rails and start building things.