So the idea is that people can continue to test patch functionality- and API-changes against a snapshot while HEAD continues to flux out. And when the patch looks RTBC, it's re-rolled one last time against HEAD. Fixing all the function conflicts, etc.
Also, people can begin porting very early. Then they can have a say on where the API needs to change. This currently needs a release cycle. That's not good. It's fairly impossible currently as there is nothing to hold on.
How often? I do not know. Daily or weekly or even biweekly... just something, please. Discussion already at http://drupal.org/node/190901