>What would be the advantage and it which situations would that advantage show?

E.g. for futuristic weapons with 2 different firemodes.
Furthermore when using left/right mouse button for different items, you can activate both of them much quicker. When no activation is available, e.g. rock or wood, the could be thrown away with mouse and strg+mouse.

But otherwise i agree with you. Throw is used much more often than secondary activation modes.
Perhaps I just need more practice to stop throwing away sword, shield, spear etc.
(Because in CR it was not possible to throw away e.g. bow, spear and two-hander without doubledown-throw. Some kind of small barrier against accidentally throwing them away.)

Some other things regarding inventory management:
I don't know if it already exists, but a way to reorder the stuff in the inventory slots would be nice.
Also I don't understand why you can select empty slots while scrolling with the mouse wheel instead of only switching to slots with stuff und don't allow scrolling while you only have one item.
Perhaps it comes with the experience, but in melees, especially in time-critical situations I often select the wrong items or empty slots when using the mousewheel.

Other things I miss at the moment is an intuitive and effective fast switch between 2-3 selectable items (not necssarily between the last two items you selected) and the nice look of showing e.g. the shield together with the sword. (Like the knight back in CR)