The beauty of using YAML is that it works well with simple shell tools like grep and also diff's well because a change of one property involves a change of one line. XML is too often stored without linebreaks, not to mention what happens when you add linebreaks and then diff that.
Re^4: Open Human Readable Computer Aided Design file format
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Obviously, line-oriented tools, such as grep and diff, are mismatched to XML. A proper XML toolset operates on the data structures, not on the raw text.
The "one difference, one line" feature is nice... but it doesn't include any context, i.e. where in the data structure the different element is; you'd have to do something else to reveal that (e.g. load it in an editor). An XML diff'ing tool can report such context, because it knows it.
Don't misunderstand me — I'm not defending XML. I know it has its problems. But — unlike yaml — being gratuitously different isn't one of them.
Ahh, but one-entity, one-file gives most of the context you could want. Currently, I've just started a huge pile of work on defining a new hub for the über-converter, so I'll see how well this holds up moving into the chromista hub design.
This example is a little old, but maybe gives you a good idea of why YAML is a Good Thing™ in this case.