Usefulness

Which goes back to what I was saying earlier about not having to constantly lather your pets in poison (though that would be cool). I tend to train my pets to around lvl 50+, so that accounts for why my snake has a better chance at poisoning whenever I'm on my alts. The venom sword is a more expensive yet effective alternative to snakes and poison lathering, but not everyone has one (anymore).

Edit: forgot to mention that although you technically have unlimited "hits" using a snake (until it either gets killed or runs out of summoning time), poisoned weapons do have 2 huge advantages: you can use them any time (can't summon mr./mrs. Snake while pulling that cart around), and splash/AoE skills like windmill, crash shot, r5+ smash/magnum shot can technically poison everything that gets hit, and it would only count as 1 use. I've tested this out while running sulfur spider. Tl;dr use a snake for 1 opponent, and a combination of a poisoned weapon and an AoE skill (That isn't magic or alch) for multiple enemies.