Yes SDM-admins who're not a site-wide admin is necessary, but do make the site-wide admin able to admin the SDM as well!
I get annoyed by some of the ACS 3.x modules that make the sitewide admin add themselves to a special module admin group before they can admin the module (user groups comes to mind). It's very inconsistent and frustrating.
It's been "fixed" in ACS 4.x - unfortunately by making ONLY the site-wide admin able to do certain package-related admin tasks!
