The only situation I can think of where I would not take the poison would be if I played as super duper rightous character who doesn't want to take the poison, but is also to goody two shoes to lie. Then again, a person who would worry about lying, is also a person who would likely be worried about becoming a monster and killing everyone.
It's one of those none-choices for me. Whatever my thoughts and intentions why wouldn't I take the poison? Promising to take it doesn't mean the character is willing to take it. You either side with her, or you make the promise to get away from the weirdo druid as fast as possible (and free, powerful poison for your enemies in the inventory!) It's definitely one of those moments I wish for Planescapes' "I promise (Lie)" option for roleplaying purposes.
