Is an ISO/IEC 15504 conformant model absolutely required to start a software process improvement program?
Certainly not. It is not a panacea and all models are incomplete and leave room for improvement. Nevertheless, ISO/IEC 15504 as an international standard represents a standard whose robustness and usefulness have been verified by hundreds of companies around the world. Using a standard with these characteristics as a reference to guide your investments in software process improvement is certainly a good decision to minimize the risks and to maximize the benefits.