When you do this in Safari, it makes a new window with the tab's content. In the Dock, it removes the item from the dock. Since we don't support multiple Inspector windows for the same inspection target (yet), we should make this interaction close the tab.
<rdar://problem/29952355>
I think the likely result would be more unintentional tab closing, and more user frustration. The current direction is to reduce the number of ways to add/remove tabs, not add more.