We didn't move as such - a plain text version of the API for dumb clients (like shell scripts and humans armed with curl/wget) makes sense, as does a web-friendly JSON/YAML option. XML can be reserved for the hardcore stuff but if you've got to parse XML anyway you may as well parse Atom - trust me, a lot of the stuff you end up trying to deal with (such as categories, links, etc.) are already done for you and there's a heap of code and developers who already grok it (e.g. Google GData APIs).
Basically I'm trying to extract the best points of all of the APIs and leave the cruft (such as in-band authentication) behind.