As it stands right now, the stat.useItem criteria for scoreboard objectives can be applied to countless items that do absolutely nothing. For example, I could create an objective called "diamond" that uses the criteria stat.useItem.minecraft.diamond, but diamonds have no right-click function, using them to break things is exactly like using your hand, and the only thing you can do with them is craft them into things. So basically, that scoreboard objective would never increase naturally; it would be exactly the same as if I had used the "dummy" criteria instead of stat.useItem.
I often build command contraptions where I create custom items, and I almost always have to end up using a carrot on a stick for everything, because it's the only item in the whole game that has a right-click function that does nothing, and therefore the only item that it's convenient to track with stat.useItem (and in the rare case of a player riding a pig, there are NO items with this feature). Like I mentioned before, there are countless items that can be applied to stat.useItem that do nothing, so why not let the objective track whenever you right-click those items? So, my aforementioned "diamond" objective would tick up by 1 every time a player right-clicked with a diamond in their hand. Does that make any sense?
If this were implemented, a whole world of new possibilities would open up for mapmakers. Imagine having a blaze rod that sets everything on fire within a 5-block radius whenever you right-click. Or a diamond that teleports you home whenever you want. Currently you can do these things with a carrot on a stick as I said earlier, but it doesn't make that much sense, and plus you can only have one of these actions at any one time. You get the idea.