In my SOA versioning, compatibility & composability session at NNUG this week, I stressed the importance of recognizing the ripple effect that incompatible or semantic changes to service contract artifacts will have. This illustration captures how versioning an artifact will affect upstream artifacts:
The ripple effect is a bottom-up effect, and an incompatible change on e.g. a customer address schema will cause an explosion of new versions of all affected contract artifacts. In the end, the change will propagate even to the service consumers.
Having a versioning strategy let you control the effects of the inevitable changes. Using a compatibility policy will help you alleviate some of the negative effects of versioning.