They are absolutely not necessary and with myself as an example, i fasted for 30 (predominately), 30 and 36.5 days without them and comfortably. At various points in these fasts, my body produced bowel movements all on its own (or in some cases not at all) and my colon rested intently and undisturbed in between. Each time at the end of the fast, my bowels performed famously and most commonly very soon after the mere introduction of juices.
I am of the belief that the colon really does not wish to be disturbed by our own forcing measures when it has things completely under control and is enjoying a well earned and profound rest. I reiterate that if our body really needs to let something go, it will. When i think about disturbing a hibernating colon, I imagine waking a hibernating bear and the disharmony and frustration this would create.