"Military" doesn't mean only weaponry or sensors, it includes all of the requisite logistics, supplies, stores to sustain a base as well.
Why would one need a civilian population be necessary for housing and stores and basic necessities for military personnel on these islands, rather than just having military associated housing, stores and basic necessities and other supplies?
Having a civilian population on any small island makes sense if it is intended for planting the flag for sovereignty related reasons, or if an island itself has a natural self sustaining way for a civilian population to exist (tourism, or existing in an economically self sufficient way that exists as an outgrowth of a major military base).
The problem is those islands in the SCS are relatively small and any civilian population would be small and token and more likely to impede and complicate the primary military/security focus of the islands to begin with. Considering the small size of those islands and the fact that those islands are also high value military targets for everyone in the region, I would also be surprised if a civilian population would willing go to live there.
In the long term, the military utility and survivability of those islands will be more reflected by the primary regional capability of PLAAF/PLAN/PLARF fires and air and naval projection, rather than only how survivable or repairable those islands are or how many missile batteries they have. Because if those islands are forced to defend themselves without a comprehensive defensive screen projected from ships and long range land based air from the mainland, without supporting offensive fires to target the fires platforms of an opfor, then those islands may as well be just expensive targets as they will be cut off from resupply if the PLA can't ensure air and sea control, and will simply be defeated in detail.