I've implemented this on two sites now and I have some feedback. First let me say I love the new default look and the CSS sheets.
But in terms of the masters...First the names are not good. Blank implies to me something you should fill in. However the blank master is the one you are least likely to need to change. Maybe "setup-master"or "top-master".
Second I am tempted to recombine site-master and default-master. Perhaps this is because my use cases are different then yours.
I am roughly dividing the purpose of graphics into two categories.
Branding: Usually affects the top and bottom, logos graphics and colors (CSS) of a site. This is currently controlled mostly in "site-master"
Information Architecture: Controls how you use the site, where and what the tabs are. A dotLRN based site often has very different menu structure then a non-dotLRN site. In my use cases this varies significantly from implementation to implementation but not much from subsite to subsite. Currently this is mostly controlled with the "default" or "group" master.
Currently there is a parameter to change "default-master" between subsites. However,
in my experience/use cases changes in templates are usually done to support changes in Branding. In other words one site is used to provide basically the same functionality to different groups within an organization or different organizations.
Most of the changes are therefore in the middle "site-master", not to "default".
Are you implementing a lot of another kind of use cases, where the branding stays the same and just the navigation switches between subsites? I have seen this in the past but its not at all what I'm seeing these days.
Right now I'm leaning towards combining "default" and "site" master into one. I don't think we can predict which part is likely to change and which is likely to stay the same. Each layer of template adds to the learning curve and debugging time. Maybe call this combined template "customization-master" and the other one that we don't want graphic artists touchng: "top-master".
A site with a number of different graphic designs would have more then one "customization-master" but hopefully only one "top-master". Ideally the "customization-master" has one css linked in that modifies all the previous CSS sheets and can be different for each different design. We also need a way for the graphic artist to control what directory all the pages look for graphics in so the page graphics can match the outer design.