Originally posted by Bravely Bold Sir Robin:

Sometimes 10 poison damage can be the difference. Countless times draugr fang poison would kill the drake after 2 arrows, instead of waiting for a 3rd.

That's a bit of an illusion, since the poison damage is part of the total weapon damage anyway, it's not an extra added damage or effect. For example, Draugr fang deals 56 pierce and 20 poison damage (by default, average skill) at max upgrade, Spine Snap deals 76 pierce damage plus 10 spirit damage at max. If we're looking at arrows, poison arrows deal more total damage over needle arrows, but the poisoning doesn't stack, so if you need to take two shots anyway, the poison won't make a difference there; some of its damage potential may even be wasted in case you need to take the target down fast instead of being able to wait for poison to take effect. Other than that, it just boils down to damage type resistances and minute effects like staggering (carapace and needle arrows cause vastly more staggering, but the effect is negligible or nonexistent against powerful foes).

So poison damage (and to lesser extent, spirit damage) is useful if you're going to make one attack at a time and then wait for the poison to run its course. You'll be wasting the weapons damage potential if you refresh the duration, since poison doesn't stack in duration or intensity AFAIK. With the jotun bane this would be accomplished by parrying an enemy, and using the secondary attack on them for maximal damage, then defending or recuperating. But there exists a lot of different combat situations. You may not have enough time (or stamina) to execute the stronger secondary attacks or wait for the enemy to die from poison, so the end results are again dependent on enemy resistances to damage type and the type of engagement in general. But, since most enemies, especially stronger, numerous enemies in the later biomes, are organic and not resistant to poison, and the total damage of jotun bane axe is like 10-15% higher than black metal axe, while using alternate materials (iron is rather plentiful in mistlands, black metal is not, but players may have large reserves of both), and both weapons have same durability, it may be worthwhile to switch to jotun bane even if its stamina use is slightly higher and you already have black metal axe. You can still use the previous axe as backup or give it to a friend who is still progressing. It may also be valuable at future stages of the game, and should make for a useful primary weapon against seekers thanks to high share of the elemental damage type, especially if you don't want to give up on the utility wood chopping ability of axes, although personally I prefer mistwalker sword while working in mistlands.

Poison can also be effective in revealing enemies through fog or mist since the green cloud effect is very visible even in adverse visibility conditions. Draugr fang and especially poison arrows can be very useful against gjall, and jotun bane might be more useful against soldier seekers than regular swarmers.