> One time an audience member asked me "Why would you ever want 
> to store the
> complete XML document if you're using an SQL database?"
> 

I've used another example to answer this question.

Let's say you're using XBRL for internal reporting in your company. You get
XBRL reports from your subsidiary companies, and you extract the data you
need into your relational database. But there's other stuff in those reports
that doesn't fit in your relational database - it's information that's been
reported, because your subsidiaries are using different accounting rules,
but which you don't actually need right now. What are your options? You can
extend your relational database to hold the extra information - that's extra
cost with no immediate benefit. Or you can throw the rest of the data away
(you're not serious??). Or you can store it somewhere, as XML, just in case
you need it next time the auditors ask some awkward questions. Most people,
I think, would choose the last option - not because it's a legal
requirement, but because it's the option with the best cost-benefit.

Michael Kay
http://www.saxonica.com/