There have been many threads on this topic. For example, people want to figure out the ideal mix of Blitz and Plague for the Vanaduke fight. Last I checked, the consensus was that three people should all use Blitzes, but four people should use three Blitzes and one Plague. This is the kind of in depth stuff you can find by searching the forums.

imo, you shouldn't need a plague needle, period.
1. the poison isn't consistent unlike venom veiler. Even dread venom striker has more consistent poison chance.
2. the damage difference is about 50 - 90 damage per bullet charge depends on monsters, even when poisoned the damage gap is only closer about 20. (These numbers I vaguely remembered back before 10% gun damage boost and 20% charge gun damage boost update, I bet the gap is even bigger now)
3. 4 pure blitz at vanaduke has about the same killing speed as 3 blitz + 1 plague.

why bother poisoning when you can one-shot anything with blitz? Gremlin and undead also dies in one-shot blitz charge.
The only exception is shadow lair, but I doubt plague needle would be useful at shadow lair.

plague is good for jk
but from what ive heard blitz is the only way to go for fsc/koa/lv/whatever you want to call it

I got a Blight Needle because the recipes were basically free due to how much Blast Network I play.

Also, since I'm trying to have Mr-Baldy use greenish equipment, Plague is the best choice for me purely because of cosmetics.
(Bytheway, Plague is better than Blitz for JK but it's beaten by Toxic Catalyzer.)

Think of it this way, do you like PvP?
39kcr = 3-5* recipe for blitz
300Kc's = 3-5* plague recipes

Krogmo coins cost approx. 30cr each (if you win and lose equally... if you win more, then they seem free, if you lose more, they're more costly)
30cr x 300 = 9000 cr That's right, 9kcr for 3-5*. DEAL WITH IT!

So you're getting a 5* weapon for 30kcr less.... Let alone the weapon being decent in PvE (Poison also causes hits to hurt less, making tanking them (see also: Vana going into Mashfest mode, trojan multi-smash/charge up, lumbers cornering, devilites locking on you all at once, etc.) easier)

It's also been mentioned that the Plague Needle has a "20% cooler" factor. Similar to Boltbrand (I could care less about ugly 5* brandishes) and Electron Vortex.

The PvP weapon lines offer three factors, cool factor because less people use them, versatility to the line in general (there are just some areas where poison may be preferable to damage, shock to fire, EV to GV, and STUN to freeze), and a fun method of acquisition. Granted, while each one respectively has their own particular niche (stagger storm for beast levels and the recent Legion of Almire/ mechaknight clouds, Plague for being a more well-rounded status-dealing variant of the Blitz (troll them gremlin menders and silkwings you bad knight you), Electron Vortex for being all-around better in general CW running (save in shock strata), and Shockburst Brandish for being a contender for best brandish ever made). They offer a cheap alternative that makes things easier on knights.

In my opinion, Plague's more beast than Blitz, it makes what should be painful moments (like when your wonderful blitz charge doesn't get completely into mr. horseface, but plague makes it (more oft than not) poisoned and when they smash into your friend they hurt the friend less). The plague CAN OCKO most anything the blitz can, you just gotta make a few more bullets hit. The poison rate has been kind to me thus far, it will proc on avg 1/3 the bullets it puts out. If you're a rabid, machinegun-toting Ramboknight, most the field gets poisoned. Which, while not many people care, does indeed help in various situations. Did I hear a "Stay Close!"? Watch your mender buddy receive PTSD as everything falls apart around them. Silkwings eliminate single targets pretty well too.