When you want to move a folder to the Finder sidebar you would have to have a Finder window open showing that folder. So why should Apple provide a context menu item for moving a folder to the sidebar when the user just hast to drag the folder a bit to the left and drop it there?
Not sure what to answer here.
I don’t like to drag and drop things. 
I see this only has a strange option instead of using a menu to do the same.
On general, the right mouse menu contains nearly and and all options you could use on a file or folder.
Why no “Add to favorites”, as is possible with copy or move too?
And you give the best example for this: You need to have both Apps open, at best side-by-side to use drag-and-drop … this is so complicated and slow, compared to selecting one or more items and use a right-mouse menu (or top-level menu) for any required command.
It’s a question of speed and convenience …
DEVONthink on the other hand …
About DT, I do not understand what your are saying …
I meant to offer a right-mousebutton or top-level menu entry with “Add to favorites” for any DT item. This way, we could simply tell DT which folders to add to ther favorites, be them any inbox or any folder … just the same procedure for any such item and not different ways, like it is done now.
But regarding your goal: You can open items from within DEVONthink with other apps but you have no access to the database content from the outside, unlike DTTG which can be used as a file server.
I know, and this is what I was asking for in the topic I mentioned: To offer the DT as a regular Finder folder, like this is possible with DTTG.
Would be fantastic.
And no, I did not mean indexed folders, I meant regular imported DT content - and accessing them over a Finder folder from other Apps.
I tried to explain that, but often I am unable to explain what I mean to achieve… to quote again: https://discourse.devontechnologies.com/t/accessing-dt-content-as-volume-in-finder
Thanks