Well, to answer the question in your title, never.
As for reasoning:
I import SoapUI directly into my Java projects. It makes it far easier for debugging, viewing your raw requests and since it stores the requests, replication of any defects found. I just have my Java project build SoapUI projects and run from there.
It builds a property file that can easily be shared, a SoapUI project file which allows the user to view everything through SoapUI's UI by loading the project file and, most importantly, if I am behind on an update and there is something off with my code, they can still create the project from the updated WSDL, import into SoapUI and make the edits required until I have time to update my code.
All of that being said, no one tool is ever the only thing you need. But SoapUI is one of the better utilities I have found for testing APIs.