Before the Viper fights the Mountain, they show his squire clearly apply the poison to the spear. Which is why I take it they might like to use it, but they have to make the effort to make sure it's used before each fight. And in the case of that original weak ass Sand Snakes fight, it made zero sense for ol' girl with the great tits to poison her sword/knife ahead of time. Since I doubt she was intending to use time-acting poison on the royal guards or Mycella.

The only context it would make sense is in relation to Bronn's "The Dornish are fucking crazy" line. Which, as I explanations go, I guess is okay, but still weak.