No. I don't think WoW even cares about the xmlns attribute (I've seen plenty of addons that don't include it but do, presumably, work just fine), but if it does, Blizzard certainly would not let you point it to random third-party URLs.
ehmm I meant the xsi:schemaLocation attribute instead of the xmlns attribute
since blizzard doesn't provide an online mirror themselves (except for the outdated download), wouldn't it still be better to just point straight to the third-party URLs?
and as far as I can see, nobody seems to really care about it ..