I'd make a snide comment about Final Fantasy, but loads of RPG's are guilty of this. If anything, it's more of a surprise when a game (besides Pokemon) lets you use status ailments.

Apparently when final bosses DO have unchecked (or intentional) poison vulnerability, players tend to break the game. Like Mother 1+2 (not "stock" Earthbound), you can bring an item to poison Giygas and defeat him without praying.

I remember some web-based game where the optimal way of defeating certain superbosses was to use an otherwise useless item that inflicted stacking poison damage, and managing your survival while their HP chipped away.

I also forgot the franchise (maybe FF itself?) where some designer or person with deep knowledge of mechanics pointed out that every boss is vulnerable to one single "mook-grade" status ailment, like Earthbound's Carbon Dog being actually vulnerable to PSI Flash's chance of insta-kill. not a good idea to try this again when it's reborn as Diamond Dog