v=spf2 xml=http://www.schmerg.com/spf.xml
or
v=spf2 xml=http://www.schmerg.com/spf.xml mx -all
... meaning "if you don't want to pay the price of setting up a full TCP
socket, talking HTTP (or whatever URL is specified), and parsing XML,
consider my record to be empty (first case) or 'mx -all' (second case), but
if you're willing to pay for more detail please retrieve the specified more
detailed XML record (which, if retrieved and considered, replaces rather than
supplements therest of the SPF record in the DNS TXT record)".
None of the arguments against XML are addressed by this format, except size of
DNS records. And, in it's place you've added additional problems relating to
HTTP.
Finally, until the SPF mechanisms are made extensible (beyond the 'exists'
record), XML doesn't even have any advantages (beyond Microsoft saving face).
Michael R. Brumm