Dave Orchard questions some of the orthodoxy concerning when to use URNS versus when to use HTTP URIS.
As Dave says, context is the critical thing. Context is king. The notion that a URN is somehow "better" because HTTP URIs tie things to places/versions/whatever is simply not true out there. Just looking around at the way HTTP URIS are really used on the Web shows that it isn't true. Context is king. Identifiers are dereferenced at a time and a place with an abritrary rich resolving context. The act of resolution can - and frequently does - take full advantage of all that contextual information.
Any would-be rigid designator, needs a state of affairs to be dereferenced against. HTTP is a well known resolver of rigid designators given a state of affairs:-)
Most ironic in all the MMTT URN shenanigans is that so many URN resolving schemes end up doing a quick shimmy to convert the URNs into unique HTTP URIs :-)