Sorry for my late reply. Have been traveling.
I see where you are coming from, and if I understand you correctly what you are doing is allowing people to specify the retrieval of all attachments with a specific tag that is also attached to a specific post_type (i.e. 'post' or 'page' or 'custom post type'). But I wonder if people have actually needed that sort of specificity?
No, you are not understanding. What Media-Tags does is allow the user to tag the media items. Media items are always of post type 'attachment'. This is per WordPress core functionality. And post_type 'attachment' items are always associated with a parent post type like Post, Page, custom post type. So when someone uses the Media-Tags template functions to query all the attachments tagged a certain way it is the same as you showing all Posts in a specific tag. But this is specific to the attachment post type.
See WordPress treats attachments different than Post, Page or some custom post type. If you were to register a new taxonomy 'Events' and associate it with Posts you would see the new menu item show under the Posts menu section.
But for attachments this does not function the same. If you register a taxonomy and associate it with the 'attachment' post type you will not see a new menu under the Media section. And what you will see on the Media item form is a simple input field where you can enter something.
So what Media-Tags does is provide an interface where the user can tag Media items (post_type = attachment) is a familiar manor. Again this is specific to the attachment post_type.
From what I see you writing you want to associate Media-tags with all post types Posts, Page, whatever, You wouldn't do this with the post tag or category taxonomies.