If your client code is tightly bound to a client library for a particular protocol implementation, it can become hard to maintain.
For example, if you move from Apache SOAP to Java Message Service (JMS) or enterprise bean, the process can take a lot of time and effort. To avoid these problems, you need a protocol implementation-independent mechanism for service invocation.
Related concepts
WSIF - Web services are more than just SOAP services
WSIF - Incorporating new bindings into client code is hard
WSIF - Multiple bindings can be used in flexible ways
WSIF - Enabling a freer Web services environment promotes intermediaries