When thinking about applying the Principle of Loose Coupling to a SOA, folks naturally gravitate towards and understand the separation of Interface from Implementation and the attendant corollary of information hiding such that the Interface is not a direct reflection of the  implementation. 

Another equally important aspect of Loose Coupling is the separation of Infrastructure functionality from the Business Logic.  There are always going to be cross-cutting concerns that apply across service implementations in a SOA such as Authentication, Auditing and Logging, Transformations, Crypto and more. Now, each service could implement these aspects on its own, but in an Enterprise setting where one needs consistency of and visibility into multiple service implementations, it is important to abstract these aspects into the infrastructure and not leave it up to each service on the how and when it will implement this functionality.

If you put the right type of SOA management infrastructure in place, you have the ability to centrally manage the creation and versioning of  policies and to push out these polices to distributed Policy Enforcement Points. This type of infrastructure is critically important to the Run-Time Governance that is needed in a SOA as it provides the ability to administer, monitor and control your environment in a policy driven manner.  Of course, Governance goes beyond just these run-time aspects, and in building a service eco-system you want the right folks to be in in charge of the right pieces e.g. You want the Security folks to be in charge of defining security policies and verifying compliance with those policies. The abstraction of infrastructure functionality that can be commonly used and leveraged gives a mechanism for doing just that.

The reality of any Enterprise is that there are going to be both "Rock-Stars" and Lounge-Singers who are going to be building Business services, and as such they should be spending their time focusing on business functionality and not on core infrastructure capabilities that are generic and need to be implemented in a consistent manner.