The problem with XML is that angle bracket expressions take up too much space because you need to duplicate element names. I don't love JSON, but there is a reason I recommend OJ.
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The problem with JSON is that the keys take up too much space because they are duplicated. I don't love BSON, but there's a reason why I recommend bson-ruby.
And I could keep going... ;)
The benefit of using YAML is precisely that there's meaningful whitespace. Different strokes for different folks.
