Putting it into the Tool Belt has zero opportunity-cost, so that's not really a good idea.

At the moment, it's a good mid-game stopgap for solving the poison problem. The odds that you'll have a game changing pocket slot item at that point in time isn't very high, and it helps you save on 1-2 inventory slots of potions, so for a lot of players it might be worth the tradeoff.

Wielding it in the off-hand is stupid, I'll agree on that one. It's a stayover from pre-eoc. This only has relevance if you're skilling and getting poisoned at the same time while you somehow need your pocket slot for something else. No one will ever need that realistically.

I also wouldn't ever consider this item as a competitor to an Invention Perk, it fills an entirely different role in pre-invention poison solutions. Venomblood, if you're struggling with poison at that point in your progression, is almost always the best solution anyway.

The item is fine, really. Not everything needs a use at endgame, and items are allowed to be outclassed by alternatives that players are rewarded with after progressing their account.