In Blessed are the Limited I said that a key to the Beatles productivity was that they had far less options than even your average independant musician circa 2011. Songwriter/blogger Nicholas Tozier agrees -
Constraints lead to creativity. Restrictions often lead to ideas—they force us to stretch, to grow, to improvise, to tap new resources and to work around the limitations. I think as much about new, interesting constraints as I do about new song ideas. I love throwing a wrench in my own gears, defying my usual creative habits. A new kind of process leads to a new kind of song.
Listen to Nicholas Tozier: How the Haiku can Improve Your Songwriting or read the transcription.