What are the pitfalls for Integration without middleware

Summary:

What are the experience/pitfalls if we develop and load stylesheets in OTM for inbound/outbound xml transformation, then integrate to external system, without middleware?

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I understand OTM has the capability of having custom stylesheets to transform inbound/outbound xmls into format required by external system. In that case, if external system supports web service (soap), won't that totally eliminate the need of having an integration middleware? Any experience sharing will be appreciated.

Version (include the version you are using, if applicable):

24C

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