OE classic could generate both versioned and unversioned scm snapshots, OE core (or rather bitbake/fetch2) can only do unversioned and versioned per SRC_URI.

Versioned tarballs serves 2 purposes:

1) an easy way to check GPL compliance, an 'ls | grep' is enough
2) deterministic mirroring

A quick hack to have a global switch for it can be found at:

http://patches.openembedded.org/patch/11971/

From a mirror consumption perspective a workflow could be:

1) see if rev exists
2) try pulling
3) try versioned tarball
4) extract tarball into tmpdir, fetch objects into DL_DIR/git2 repo

That would stop things from being blown away. It does allow the  
versioned snapshots to differ, but they will always have the rev they  
were versioned with.

The order for mirror searches could be:

1. Specific Revision X tarball
2. generic mirror tarball
3. git clone

For more info see the thread at http://thread.gmane.org/gmane.comp.handhelds.openembedded.core/7888