Current features that use SMS include hold-ready-for-pickup notifications and a "Send Text" action for call numbers in the OPAC. If this setting is not enabled, the SMS options will not be offered to the user.  Unless you are carefully silo-ing patrons and their use of the OPAC, the context org for this setting should be the top org in the org hierarchy, otherwise patrons can trample their user settings when jumping between orgs.