Tim Bray has written a good post about XML and how it is not a great idea to begin writing your own XML standard in order to store and transfer your data. He makes a good point by referring to Metcalfe's law of network value and applying it to the realm of XML standards:
"The value of a markup language is proportional approximately to the square of the number of different software implementations that can process it. I could argue this from theory but would prefer to do so by example: HTML. RSS. PDF."