Is the IOSA useful even if not all airliners are IATA members?
IOSA is basically an audit program under IATA. It is not a regulatory regime and as such is different from the FAA, EASA etc, which have force of law. In fact, IOSA is only one of the various audits offered by IATA, which include:

IATA Operational Safety Audit (IOSA)
IATA Safety Audit for Ground Operations (ISAGO)
IATA Standard Safety Assessment (ISSA)
IATA Fuel Quality Pool (IFQP)
IATA Drinking-Water Quality Pool (IDQP)
IATA De-Icing/Anti-Icing Quality Control Pool (DAQCP)

IOSA deals with the operational management and control systems of an airline. The IOSA standards and recommended practices are derived from the ICAO recommendations. From the IOSA  Standards Manual

Sources for IOSA Standards and Recommended Practices (ISARPs)
The safety and security requirements published in the Annexes to the Convention on International Civil Aviation (ICAO Annexes) are the primary source for specifications contained in the ISARPs. Safety and security requirements in the ICAO Annexes used as the basis for ISARPs are those that
are applicable either directly or indirectly to the air operator.

However, these standards are not regulations (as are those defined by FAA etc.):

ISARPs contained in this manual have been developed solely for use under the IOSA program and contain the operational criteria upon which the audits are based. ISARPs are not regulations. (emphasis original).

The usefulness of the IOSA to airlines if from the fact that it is a standard recognized by most other airlines- instead of going to different auditors to ensure compliance of standards, they can agree on a common standard (also IOSA is required for IATA membership).