While the Blitz needle is the gun of choice for many knights due to the incredibly high power of its charge attack there may be reason to own the other autogun lines
The Fiery pepperbox has no strengths against any monster type but no weakness as well, this combined with the knockback, flinch rate, and decent DPS make for a gun that is more suited to pushing monsters back and chipping away at groups with an easy way to spread fire status (there are many weapons that can spread fire better but most if not all 5star fire weapons have elemental damage and do less damage to beast and gremlin)
The Blight needle has the same damage type (piercing) as the Blitz but even with the poison it cant match the DPS, so that leaves the question, why get one? First poison has more effects than lowering defense, it weakens attack and reverses healing which means that powerful monsters like vanaduke dont deal as much damage and menders are dangerous to poisoned monsters.
Both the fiery pepperbox and the blight needle make excellent support weapons for teams where safety is a concern.
But for you who love to see dozens of big numbers pop up when you unload into a monster's face, please continue to do so.
Feel free to critique my overview and provide me with suggestions on what you think I did right or wrong
In this game, there are several cases where you can trade a little damage for status: Acheron vs. Obsidian Edge, Nova Driver vs. Storm/Magma/Hail Driver, Blitz Needle vs. Plague Needle. Sometimes the extra damage makes no difference, in which case you should go for status. But sometimes the extra damage is just what you need to one-shot-kill a monster.
In the case of Autoguns, you want the extra damage so that you can one-shot-kill slag guards, kill Vanaduke in fewer charges, etc. Expert players don't care about lessening his damage because he never hits these players.
I do agree with your general sentiment, that many players could help their parties by spreading statuses such as fire and poison. This sort of support role is not very popular, though. Perhaps it requires a certain amount of maturity and selflessness.
Edit: By "support role is not very popular" I mean that not many people enjoy playing it themselves, even if they appreciate having other teammates play the support role.