Of the 17 original authors of the SOA Manifesto (including yours truly), Anne Thomas Manes was one of the most -- if not the most -- powerful and influential voice shaping the formation of the final document.
Anne just posted her analysis of the Manifesto document, which is definitely worth a read if you seek to understand the thinking behind the wording of the core values and guiding principles.
Anne made many clarifying points, and I'll highlight a couple that bear repeating:
On "Intrinsic interoperability over custom integration": "To some degree, I'm surprised that we were able to get the ESB vendors to agree to this value statement. If you build services that support intrinsic interoperability, you really don't need an ESB. Of course you might use an ESB as a service container (not just as a mediator), in which case you can use the ESB to expose an intrinsically interoperable service. I'll just point out, though, that RESTful services are more intrinsically interoperable than other types of services."
On "Shared services over specific-purpose implementations": "I find this value prioritization is ... missing in many organizations. Unfortunately, the impediment to using shared services is a cultural issue that's very hard to overcome. Many organizations suffer from chronic 'I'm special' and 'not invented here' syndromes, both of which often represent trust issues."
Anne also points to the fact that we use the term "shared" rather that "reusable" in the above value statement. She observes that "we had pretty strong consensus among the authors to avoid the term 'reusability.' We prefer 'use' rather than 'reuse.' ... Reusability is a tricky subject. A few years ago, I thought that service reuse was fundamental to the SOA value proposition... But I've backed away a bit from that focus on reuse. Designing for reuse is expensive, and many services will never be reused."
The potential for sharing of services, however, should be considered at the initiation of a project, and these points are embedded with the Manifesto's value statements elsewhere as well. As Anne puts it: "Designers should always consider whether the services they are building could have value to the organization beyond the scope of the individual project. If so, they should design the service for reuse by concentrating on getting the granularity right and by adopting corporate standard data models that enable intrinsic interoperability."