Would anyone really characterize the villains in Die Hard as 'terrorists'? What exactly is terrorizing about drilling into a vault in a secure building in order to steal money? How are they not just robbers?
The point is, spoiling it adds nothing to the conversation: "I watched the movie and you got this plot element that, while important to the film, is completely unimportant to your point ... wrong!". On the other hand, not spoiling it lets anyone a bit too young to have seen it the first time around not have their experience of the film permanently altered for the worse. There is no contest. Don't spoil things, even if it's old as hell. It really, honestly ruins things for some people who are a bit behind the times and care about this sort of thing.