Obviously, I think you can build services using a variety of frameworks, and so SCA is certainly not a requirement for SOA. That said, I think SCA has some very compelling features as it relates to assembling services through reusable components; supports a broad-range of bindings so that services can be exposed over
SOAP, REST, JMS, ESB, RMI, RSS/ATOM, JSON etc; and has a powerful distributed deployment model. I don't think SCA is all that complicated to get started with, and hopefully my book addresses that through its case studies and examples.