An update from a heated debate context:
Let me summarize:
1) We ALL agree XML is not the best interchange for EVERYTHING
2) Google COULD have used XML if they wanted ( or JSON looks like a better fit)
3) Using an established format would NOT have effected the actual TRANSFER
4) I admit that Google knows their stuff and has probably analyzed this that XML for the TRANSFORMATION may have been too fat. HOWEVER - they optimized, and in so doing specifically limited expandability or at least reuse.
5) Building a library to do a SPECIFIC task well is OK, but the crux of my argument is IF they spent this time creating better XML or JSON libraries, then we ALL benefit in projects that do not use this protocol.