/cast and /use can be used interchangeably for nearly every addressable slot in the game, (char/paperdoll window, bags, bank), every place has a number assigned to it. For example 16 and 17 are the weapons, 13 and 14 are the trinkets. I don't know all of them or even if the example is correct but it doesn't really matter in this context. By adding the 0,0,0 or 1,2,3 you are attempting to use an item that has no use ability, in a castsequence when something doesn't fire it would normally hang the macro, in the case of using items with no use ability, it successfully "casts" nothing and advances the macro.
For the lazy macros like the one above, if you spam the crap out of the macro it will do nothing for all the lines with a zero and then fire the first non-zero line, on the next press it will run down the lines until it hits a spell in the second position, and so on. If you don't spam in less than 0.3 seconds it will reset all of the lines and start the process over, essentially giving priority to one spell, however continuing to advance through the sequence if the spell is on cooldown, or for some other reason unable to be cast.

This circumvents the normal castsequence spell lockup, however, as it is done entirely with the in-game macro system and produces results no different than taping a popsicle stick to 10 keys and whacking away at that, people and blizz seem to have no problem with it. Just realize that their stance can change, or they can disable the ability in the future.