There isn't a hard and fast answer to your question. But I'd say rarely, because you lose the power of the database in keeping data integrity and the ability to query against the data etc. That said, like most things, sometimes the benefits of storing the XML outweigh the disadvantages, e.g.
- You want to store XML documents for future use rather than fixed data structures - for example if an application is processing XML anyway(say remote calls)
- The data's structure is open to rapid or repeated change to the point where re-definining the equivalent relational tables is cumbersome. Especially true if no querying is needed.
Obviously other circumstances exist. One situation I have seen XML store legitmately was the long-term persistance of a custom user state object that was open to rapid change as the site developed (the XML had versioning and old versions were likely). We never needed to query "how many users in state X" etc, so serializing to XML and storing that was a good option.
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