Hi,
I'll try to explain the idea of that "CTRL+select+assign" (well, and it would be a feature no other cataloging software has 
, they have "double click assign single tag", but no "multiple tags assign at once"):
Let's say, we have tag structure:
- parent 1
-- child 1.1
--child 1.2
-parent 2
--child 2.1
--child 2.2
-parent 3
-- child 3.1
-- child 3.2
-- child 3.3
We have two images: "image1" and "image2". Using largest preview size possible (we want to evaluate each image in detail before assigning tags) we want to assign:
- To "image 1" -- "child 1.1" and "child 1.2" and "child 2.1" and "child 3.3"
- To "image 2" -- "child 1.2" and "child 2.1" and "child "2.2" and "child "3.1" and "child 3.3"
The quickest way to do this would be something like this:
For "image 1" -- CTRL-select all the tags we need ("child 1.1" and "child 1.2" and "child 2.1" and "child 3.3"). Then press "some shortcut" (let's say F2), and our "image 1" has all the selected tags assigned to it. Why CTRL? It's just "usual way of selecting multiple-something" for most Windows users. Of course, it could be some "right click menu" instead of "F2". But using keyboard is much quicker (well, I'm the "old keyboard-no-mouse" school 
). The best solution, of course, would be right click menu + possibility to configure "my favourite keyboard shortcut".
Real life situation for such needs -- I'm trying to use the software for knitting patterns catalogue. So each image has something like this assigned: "sweater + woman + for summer + knitted + short sleeves". Tag combinations are rather different.