Well most of the time it is. Especially when trying to convince enemies not to fight you. They still get a chance to disbelieve your lie if they Sense Motive. They would also get a huge bonus to a ridiculous story like the tower is going to explode. Diplomacy is just another example of that and failing at it means they don't trust you. However, these are enemies. They didn't trust you to begin with. You're rolling to adjust their trust of you into the positive range. Surely they won't just roll over and take that without an opposition check to shift their reaction towards you (or at very least some Charisma silliness).