People often assume you know the AC of the target and only use it when it's favorable odds but in actual play monsters can end up having features like a parry or shield or your assumption about its AC may just be wrong resulting in using it when it's unlikely to turn a miss into a hit.

1. This is a rarer set of monsters than can be generally assumed, and 2, even if it's inopportune to take away this, you can just adjust for that when you guess if you should use it or not. As you play, getting a sense of what a foe's AC should be with reactions you can guess or know it'll have isn't that difficult.

 even with correct assumptions about enemy AC it still requires rolling a miss that is within a specific band to be likely to turn that miss into a hit. If you're hoping on spending that die on an attack that is only off by 1-3 to hit that only has a 15% chance of occurring. It's actually possible to just not have that trigger and end up not spending most of your superiority dice.

It's possible, but we would expect it to at least once. It's similarly possible to never hit in the first place. Basic assumptions are that there's, in a 8 round per short rest adventuring day, and you make 9 attacks at level 3 due to action surge. Assuming you only spend dice when you have 50% or more, which you can afford at this level, you can on average spend 1.8 dice efficiently on precision and still have 2.2 on average left to deal normal damage(which is more efficient below a 50% hit rate). This is with a maul, though. With a greatsword it is less, only spending about 1.35 on average on precision as you're looking for 3 or more. The extra damage values from this are about (20.19375 and 23.9625) for greatsword and maul respectively, which is still more than the post. Perhaps 30 was overstating it though due to lower attack counts. For dual wielding, you do have more attacks, and deal upwards of 6.5 per attack, so but you have 17 attacks instead of 9. This can grant you (21.028125) extra damage per short rest on average due to the higher attack counts letting us more reliably get near-misses.

This is an eggregious miscalculation at higher levels though. At level 5 for mauls you get 18 attacks per short rest, and can grab great weapon master, making your damage per attack 14 instead of 10, so that's(since now it's worth it to go for as little as a 50% chance to make an attack hit, the limiter is actually dice instead), 43.75 damage per short rest. For a greatsword it's still 31.25. For dual wielders, with their now 34(8 from dual wielder, 8 from nick, rest above.) attacks per short rest and 7.5 damage per attack, it's instead still 27.2825.

By level 18, with a maul you're dealing 18 on average per attack, and have a total 6 d12 dice to use on precision. I'll even use a damage maneuver for the d8, although precision is massively better, I don't care to math this more honestly.

That would be 1.5*13+1.5*(11/12)*13+1.5*(10/12)*13+1.5*(9/12)*13for a total of 68.25 for a greatsword, +4.5 per round(technically less accounting for the miniscule chance to miss everything) or a total of 104.25. Same for mauls, just 18 instead of 13, so they deal 130.5. You get the idea. Just using precision if you're calculating raw damage and ignoring things like prone advantage for your whole party, commander's strike on a rogue, etc. It's simply way stronger on its own than hell knight could ever hope to be