I've recently asked one of my developers to research some integration and middleware technologies for a project we're working on. After spending a couple days on this, he said to me "these things are all part of ESBs now" - i.e. all the integration and middleware vendors have pretty much taken these capabilities and bundled them into their ESB platforms.

"Oh you need a message bus? That's part of our ESB now." "Oh you need this adapter? That's included in our ESB."

In most cases when you're implementing an SOA, you will need some piece of middleware or integration technology. With just about every vendor making these things part of their ESB suites, you're probably gonna end up getting an ESB even if you don't need a whole freakin' ESB. That's why all SOAs need ESBs (regardless of whether or not you really need them)!

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