the hagbane is great for clearing massive packs of infantry. if you can land a hit over their heads or when they are running left to right to maximise the poison explosion radius, you will find that NO OTHER weapon matches that kind of long range horde clearing capability. it’s basically a grenade launcher.
actually it’s great for armoured specials - direct heavy shots will inject the poison and the time taken to shoot the required arrows to kill them is minimal as compared to longbow drawing. it makes a big difference when you are engaged in melee horde but need to kill that gatling that’s firing at you. heavy shot to clear the melee in front of you, heavy shots to finish off gatling. it has versatility and CC that the longbow does not offer for some scenarios.
your second point about running out of ammo - the point about hagbane is it’s extreme burst dmg capability. if i can unload 25 light attacks into a hagbane in as short a time possible, the damage is insane. i don’t want to ‘spread it out’ over the course of the fight, i want to do as MUCH damage as possible in a short amount of time, and the hagbane with any potion will allow you to do this, since the dot damage of each arrow stacks with more arrows.
that being said, i hate trolls because of the damage cut off in the recovery period after getting knocked down and temp healthed reduced to 0. any dots from the hagbane during that timeframe is just bloody wasted and i gotta wait for it to become damagable again.
chaos spawn and ratogre (and stormfiend from the back) and the best things to spam light attacks on, it’s just a mad rush to pump as many arrows into him as possible to watch that HP bar deplete really fast =)