Read the Perceptions-RFC, for more info on Stimuli and Perception system.
Stimulus concept is more Player-oriented, and Event concept is more Bot-oriented. It's really easy for a Player to interpret Stimuli, but it will be really hard to do complete Stimuli recognition for Bots. It would be far too intricated to try to make the bots realize there is an aggression when they receive a scream stimulus for example: there could be an aggression without a single scream, and there could be a scream without an aggression. To determine that, the bot would have to process a lot of other stimuli which would perhaps not be available. Each time, it would be like trying to put together the pieces of a puzzle, and that would be too difficult and too long. Events make the situation much easier to handle. That's why many Events are more specifically for Bots (they can be used indirectly for Players too).