Perhaps in a different game, those natures would be quite balanced. In pokemon however, some of those points are based on arbitrary hypotheses that do not necessarily apply to a competitive environment. While in the very first stages of the game with limited mechanics (Gen2) natures had a significant impact on the game and inevitably some were ostracised due to their impractical... nature (kek). However, with the introduction of abilities, held items, weather, and later the special/physical split, more became viable. In some cases, natures dubbed as impractical or "should never be used" became optimal for certain pokemon, or certain sets (thanks to the ability to customize your pet-battler). Let me elaborate on my thoughts here.
Prologue and General Analysis
With 6 Natures to go, we enter the real debate
Those are:
Hasty (+Spe,-Def)
Lonely (+Atk, -Def)
Mild (+SpA, -Def)
Naive (+Spe, -SpD)
Naughty (+Atk, -SpD)
Rash (+SpA, -SpD)
Which are ones I personally think have their uses, especially for mixed attackers. To whom it may concern, those creatures use both their Atk and SpA stats efficiently in battle, and are commonly found in HO teams (Hyper Offensive). These pokemon, or rather sets for certain pokemon, are a double-edged knife and it's apparent why. They decide to sacrifice a little bit of defense for extra killing potential, and believe me when I say that with proper EV spreads, these pokemon do their job extremely well. Especially Scarfers can be proven deadly with such a Nature.
Notable examples are: EQ Latios, Fire Blast Tauros, Infernape, Greninja, Salamence, Tornadus-Therian, Landorus-Therian, Garchomp, M-Sceptile, Xerneas, Azelf, M-Diancie and Zoroark
You might notice that most are frail pokemon blessed with good offensive stats in both sides, that usually function as either sweepers or wallbreakers. Since their Speed might be crucial for their efficiency, they decide to decrease a certain defensive stat. Especially things like Tornadus-T which have Regenerator, don't really care about small amounts of damage increasing if it's just a bit (since they do not invest in Def/SpD so the difference is minimal) if they can do their job properly or even exploit the element of surprize in dealing with opposing threats. After all, the game involves a lot of luck and prediction strategies so factors like a special nuke out of nowhere or a random Knock Off may even net you the game. That's even more obvious when your movepool does not allow specialization, such as M-Diancie which is forced to run Atk for Diamond Storm, or some Garchomp sets for the occasional Fire Blast, in order not to be completely walled by Ferrothorn and Skarmory. Admittedly those pokemon do not need their Def or SpD stat that much or are expected to be hit more on the other spectrum, so they are still safe.
At the end of the day, only the natures that block their own benefits may as well go extinct. Even so, if someone presents me in the future with a relatively fast mixed attacker that can excel in either Def or SpD, and has a good ability to compliment that trait, like Regenerator or Thick Fat, I might even use Lax or Gentle as its abilities. As for the 'neutral' ones, no. They can go burn in poke-hell.