the main thing I find with xml is not only is it easy to modify and maintain, it compresses really well. Most of my data files are xml and I get on average 90% compression (or more), given that the data no matter what format is used tend to unpack to the same size once loaded (i.e they all contain the same number of verticies etc) I prefer an open format for to allow me to tweek things manually.

I understand people like proprietry formats to keep the data safe, but I get round that by encrypting the bits the I want to keep safe.