If one is already suffering from an ailment/ weakness of the back, back arches might sometimes not be advisable.  But for everyone else, it is highly recommended to make a regular practice of back arches a part of ones asana practice – whether simple or complex/ advanced.  A regular practice of arching back helps in maintaining the elasticity and mobility of the spine, improves posture, improves the nervous system, is therapeutic for various physiological and psychological conditions, and brings about a sense of exhileration and upliftment.

Especially in the Iyengar system of practice, there is an immense scope for exploration of back arches: preparatory positions give a flavour and many of the benefits of back arches even to those for whom these positions might not be advisable, the final asanas have been dissected so that parts can be practiced in isolation. This brings focus to specific areas and actions.  Props make possible those positions that might otherwise be inaccessible, and they also make possible longer stays in back arches.

Note:
People with back injury/ weakness are advised to proceed with caution, and use supports where recommended.