Ah... Another Norton Commander survivor. I get my fix mostly on Midnight Commander nowadays.
Firefox allows to both run separate instances and in tabs. The ability to have more than one instance of calibre running at the same time would most likely be helpful for a tabbed interface for separate libraries as well. Just a guess.
It might be confusing with both real libraries and virtual libraries in tabs? Copy-paste or f5/f6 only makes sense between real libraries, not between virtual libraries in the same real library. Or even to a virtual library in another real library, the change might not be visible.
But I feel we are going out on a tangent here. This is hardly relevant for the current upgrades. But still fun to see that there are a lot more changes to speculate about.