It's that site-wide part that is unworkable for users in the long run.
The Image Carousel is currently the default behavior for Galleries and I believe was introduced to override the Attachment page navigation problems created when users inserted images in a Gallery that were previously uploaded and inserted in other posts. Attachment pages are post-dependent, the Carousel is not. Now you say we should turn off the Carousel site-wide if we want even one of our Galleries to link to "None".
In my opinion, this is big step backwards for Gallery usability because unlike the tiled mosaics, which can be managed on a post to post basis, we do not have the possibility of invoking the Carousel on a Gallery to Gallery basis. It's all or nothing.
So in order to use even one Gallery linked to "none" we also end up forcing users to link to Attachment pages or Media Files on their other Galleries. Again the navigation between Gallery images becomes unmanageable.
What we really need in the Gallery "link to" options are None, Attachment Page, Media File, and Carousel.
(Just a side note,
...regardless of whether the individual image links are set to none or not...
So far it has been my experience that when using the Gallery feature, there is no possibility to link individual images in the Gallery screen of the Media Manager; the option doesn't exist and the settings in the Gallery screen override any individual image settings when inserting a Gallery.)