Because it turns into a kind of Step function with particularly valuable thresholds, when you're trying to go from 2 shotting to 1 shotting, the value of making that step is abnormally high, but the total damage needed to do that is higher than any other step.
Meaning it's a lot less likely to demonstrate value in the case you're talking about, trying to 1-shot a minion wave. But in cases where it tips you over to it, it's abnormally high value.
By numbers, I'd think it'd be champs that use more faster hits that are more likely to get consistent value out of it, because they're more likely to have different hit thresholds that make you more likely to take 1 fewer hit to do something more often.
By specific threshold value, if you can get to oneshotting, great. That's the highest step value, and the biggest increase in real power. It's not necessarily the most likely, you need to already be close.
I.E., if you need to do 1000 damage, and you do 334 damage per shot, you need to do 166 more damage to go from 3 hits to 2. But once you reach 500, you need 500 more damage to go from 2 hits to 1. Everything from 501-999 only changes the hits required if damage also comes from somewhere else, but doesn't change what you're doing alone.
I think a lot of the uses of it have been in wave control tempo however. Getting rid of a few cannon minions to manipulate pushing and back timing against a lane opponent. Some of the hidden power is just the fact that it makes killing creeps a little easier. so you may have more gold than you would otherwise, but may never entirely notice where it came from.
The newer obvious thing is countering the Banner of Command and Baron minion.
Depending on what the minion health changes do, it'll resettle the value points for this for different champions wave clears at different stages of the game.
TL;DR, those occasional cases where you hit a pack of creeps and they have an invisible sliver of health left over, and you get internally irritated? That's the case where the dematerializer would've helped. And you'll probably never notice when it helps, or when it was overkill.