In a homogeneous environment it is in fact unnecessary to have a service bus at all. Routing between services and requesters may be achieved on a point to point basis. Support for security and reliability may also be implemented on a point-to-point basis. Indeed, it should be possible to resolve any issue in SOA without recourse to an ESB!
The benefit of an ESB is that it eases the process of creating an SOA. Within the boundaries of an ESB, support for multiple protocols and data transformation enables heterogeneous services to behave as if they were homogeneous. Adapters allow us to expose legacy systems as services without programming. The support for reliable and secure messaging and queuing is also available through straight-forward configuration rather than coding. Add in the availability of logging and access control for governance and ESB can be a very useful tool indeed. The downside is that it takes time and effort to develop sufficient familiarity with an ESB tool in order to achieve the maximum benefit from it.