Also thank god the people who created the script extender thought of making the function work with variables, I sure don't understand why Bethesda didn't see the need for such flexibility in their editor.
Bethesda 'didn't see the need' because they didn't have a need.
Something the Bethesda devs poiinted out a long time ago, when MW was still new: the Construction Set and the script language were not designed primarily for the players as a general purpose set of tools for us to build anything we wanted.
Instead, the Construction Set and the script language were designed as tools for the Bethesda devs to build the game world they wanted, and then they released the tools with the game as a bonus for the players.
So the editor and the script language have just the features and capabilities the devs needed to build Morrowind; the 'missing' features that people gripe about periodically are things the devs didn't need to build the game.
So if 'AddItem' and 'RemoveItem' don't accept variables in scripts, it's because the devs didn't need that capability to build the game world.
As Abot pointed out, you can use stacked 'While' blocks to add or remove variable numbers of items. If you pick the number of 'While' blocks and the sizes of the increments carefully, you can handle fairly large numbers of items without bogging the game down significantly.
...just passing through...