I think it's a little naive to expect a single technology such as web services, as flexible as it is, to satisfy all the needs of any organisation. We should be able to recognise by now there is no 'silver bullet' to solve all our enterprise requirements; as the technology advances so does our expectation of what we should be able to achieve with the technology. The ESB and WS are just different, yet complimentary patterns in the SOA fabric. The use and implementation of each will be driven by the requirements and technology focus of the organisation and the feasibility of a solution within current infrastructure constraints.
The advantage of a properly implemented ESB is that it can leverage all the advantages of WS with the added bonus of integrating with technologies that are not suitable for the web services model. Each SOA technology should be viewed as an architectural pattern in the enterprise architect toolbox along with J2EE, .NET and emerging methods such as Grid computing.
What is the business case for ESB?