default (currently) means the default website all means for all websites
Definitely the all functionality can be rolled into default, but that might be confusing. I personally would prefer the separate version, it allows treating the default site as any other.
+1 As a slight extension: all should not contain a settings.php file, default should only contain a settings.php file. If /all contains a settings file (defining databases, docroots, etc) will just cause users confusion when they try to change their local site settings. The /default folder can actually define a specific site, unlike all, which applies to many sites. This might necessitate an additional file, say "globals.php" in /all for variables that should be shared across all sites. A use case for this would be a user using a single database (with prefixing) could set the database in globals.php and delete the corresponding variable in his/her per site settings.php file. -M