If you called it top-master, that would (to me) imply that it includes the top navbar -- which is what the site-master currently does.
The reason for the split between site-master and default-master is that the site-master is (supposedly) the stuff that stays constant across your site (login/logout, home page link), whereas you'd switch default-master for different purposes, e.g.:
- Your organization's public pages may have some left-side navigation
- Your organization's "community spaces" may use the group-master template.
- Your "developer home" could use a different navigation -- whatnot.
This is in fact the use-case that's currently implemented in the CVS tree, with the default-master and group-master templates, and the ability to switch between them from /admin/configure.
Would making default-master able to dynamically pick its master template solve your problem? You can fairly easily add a parameter to acs-subsite for that.