JSON is faster and more compactfaster and more compact, so it is easier on the server, and easier to parseeasier to parse in most application languages -- while still being human-readable.
Since the API is returning data, rather than markup, JSON is a much better choice than XMLreturning data, rather than markup, JSON is a much better choice than XML.
If you insist on XML, there are a greatgreat manymany waysways to convertto convert the data yourself, without loading the API servers with extra XML baggage.
