Honestly, there's not that many non-Dark Pokemon that prefer Poison typing to Steel typing, since the only advantages is actually has are...
-Not vulnerable to Fire (Instead vulnerable to Psychic)
-Resists Fighting instead of being vulnerable
-Absorbs Toxic Spikes (Instead of ignoring them)/makes Toxic 100% accurate/heals off Black Sludge
-STAB on Poison moves. (Which most Pokemon don't actually run as coverage -Steel is more popular)
Of course for Dark types adding Steel is a double Fighting weakness vs adding Poison leaves you with only one weakness out of the two type's weaknesses. (Ground, specifically) So Dark types like Poison. But for most Pokemon you reeaaally need to hate Fire and/or Fighting for Poison to be worth considering -and in Conkeldurr's case, it leaves it with a double weakness to Psychic. Normally even strong Psychic attackers struggle to OHKO Conkeldurr due to its sheer bulk, but with a double weakness it's pretty trivial to destroy it. The main thing it has going for it is that people will be leery of tossing out a Psychic at Conkeldurr for the possibility of Dark or, less extremely, Steel, but Fighting/Dark is doubly weak to Fairy, which is bad, and the least risky option for attacking Conkeldurr is always going to be Earthquake. (It's extremely difficult to argue for Flying typing on Conkeldurr -about the only nice thing one can say about it is that it screws with people assuming Ground is a good option. Among other points, losing Stealth Rock resistance sucks. Grass and Bug make even less sense)
Even getting STAB for KOing Grass Manaphy seems an iffy utility, since Poison Jab is, without STAB
252+ Atk Conkeldurr Poison Jab vs. 80 HP / 0 Def Manaphy: 204-240 (56.5 - 66.4%) -- guaranteed 2HKO after Leftovers recovery
vs STAB being
252+ Atk Conkeldurr Poison Jab vs. 80 HP / 0 Def Manaphy: 306-360 (84.7 - 99.7%) -- guaranteed 2HKO after Leftovers recovery
still inadequate without Poisoning Manaphy. (This is the calc's standard OU Tail Glow Manaphy) And Steel and Poison both resist Energy Ball, while Steel alone resists Ice Beam and neither resists Scald. And either way you end up vulnerable to Ground, hampering you against Gliscor among others, so it's not like Poison has an advantage there. Plus Steel provides Sandstorm immunity, which is nice because of Hippowdon and Tyranitar's popularity.
I'm skeptical of Poison Conkeldurr, is what I'm saying.
-Not vulnerable to Fire (Instead vulnerable to Psychic)
-Resists Fighting instead of being vulnerable
-Absorbs Toxic Spikes (Instead of ignoring them)/makes Toxic 100% accurate/heals off Black Sludge
-STAB on Poison moves. (Which most Pokemon don't actually run as coverage -Steel is more popular)
Of course for Dark types adding Steel is a double Fighting weakness vs adding Poison leaves you with only one weakness out of the two type's weaknesses. (Ground, specifically) So Dark types like Poison. But for most Pokemon you reeaaally need to hate Fire and/or Fighting for Poison to be worth considering -and in Conkeldurr's case, it leaves it with a double weakness to Psychic. Normally even strong Psychic attackers struggle to OHKO Conkeldurr due to its sheer bulk, but with a double weakness it's pretty trivial to destroy it. The main thing it has going for it is that people will be leery of tossing out a Psychic at Conkeldurr for the possibility of Dark or, less extremely, Steel, but Fighting/Dark is doubly weak to Fairy, which is bad, and the least risky option for attacking Conkeldurr is always going to be Earthquake. (It's extremely difficult to argue for Flying typing on Conkeldurr -about the only nice thing one can say about it is that it screws with people assuming Ground is a good option. Among other points, losing Stealth Rock resistance sucks. Grass and Bug make even less sense)
Even getting STAB for KOing Grass Manaphy seems an iffy utility, since Poison Jab is, without STAB
252+ Atk Conkeldurr Poison Jab vs. 80 HP / 0 Def Manaphy: 204-240 (56.5 - 66.4%) -- guaranteed 2HKO after Leftovers recovery
vs STAB being
252+ Atk Conkeldurr Poison Jab vs. 80 HP / 0 Def Manaphy: 306-360 (84.7 - 99.7%) -- guaranteed 2HKO after Leftovers recovery
still inadequate without Poisoning Manaphy. (This is the calc's standard OU Tail Glow Manaphy) And Steel and Poison both resist Energy Ball, while Steel alone resists Ice Beam and neither resists Scald. And either way you end up vulnerable to Ground, hampering you against Gliscor among others, so it's not like Poison has an advantage there. Plus Steel provides Sandstorm immunity, which is nice because of Hippowdon and Tyranitar's popularity.
I'm skeptical of Poison Conkeldurr, is what I'm saying.