Currently all themes declare their set of regions which is global to the theme. Inwe want to make it possible for themes to define multiple layouts for the user to pick from. Those might or might not come with different sets of regions. It is possible the consensus is that multiple region sets are not to be supported for themes and one theme should have regions unified for all its layouts (which would be easier to implement both for themers and Drupal core).
However, the ultimate goal of decoupling layouts from themes is that modules could define layouts as well in future issues (http://drupal.org/project/layout is ported to Drupal 8 and is proposed to core to fulfill that role). For this to work, even if theme defined layouts use the same regions for all layouts within a theme, layouts will definitely not be unified in terms of regions across module defined layouts. This means we should stop relating blocks to themes and should relate them to layouts instead.
The list of layouts introduced inshould be able to provide the base data (labels and machine names) to relate blocks to.