Good description of the problem, really good. And yes I'll update the Menus plugin ASAP to fix/workaround the typo. But there is also a problem with the core behaviour of the Favorites menu, but only affecting such a case as you describe. I'll continue.
The code in the Menus plugin hides the comments menu if user can "moderate_comments" - which must have made sense to me after looking at the codex lists of roles and caps while writing the plugin way back. However, the code that adds the menu in the first place, /wp-admin/menu.php, uses "edit_posts" as the capability to draw the menu. The effect, or defect, is that Menus plugin is only hiding the menu from Admins and Editors while Authors and Contribs still see it. Not right as Authors and Contribs can do nothing but read the comments they see there anyway.
So that's the why.
Here's the fix in the Menus plugin:
Change the two occurrences of
current_user_can('moderate_comments')) {
to
current_user_can('edit_posts')) {
Digging deeper into the core, I think we've stumbled onto a "ticket" item for WP. The menu for Comments is also added to the little dropdown Favorites in the admin header, but that Comments menu is only added if user can "moderate_comments" - Admins and Editors. See /wp-admin/includes/template.php. Perhaps I was looking there when making the plugin last year. At any rate, that cap makes sense, because if user can only "edit_posts" they can't edit, approve, or delete comments anyway - so hide the menu.
So by default, the Favorites menu won't show Comments to Authors and Contribs while the sidebar Admin menu will - this is how it works without any plugins at all. Maybe that's a defect in the core, or there is reason to have two different caps to in essence display a link to the same page. I'll go search WP trac for a ticket and post there.
