They would add a good deal of unpredictablity, flavor and spice to the game. The current AIs tend to be a bit boring and predictable despite the huge amount of time spent on it. The "not every game" element is important. Lots of new strategies would be opened up for the human player, but none would work 100% of the time.

Let the days of vanilla flavoured games come to an end! Random events such as these need to happen more frequently in the games we play. Lets say once every 6mths or less.

Other events which I would like to see, and more often:

Assassination of leaders, scientists.
A supergenius is born, which adds a massive tech advantage for a few years until he/she burns out.
Tech stealing once on advanced
From gc1 - a precursor device is activated which increases all attributes slowly, must stop this race before they.....
Great military tactician - bonus attack defence.
Scotty the engineer is born - increased hitpoints.
megadrive increases speed
Fremen style jihad sweeps the galaxy (Dune)
Space sharks
crystalline entities
minefields
random dreadlord appearances- give them a system and a fleet
Stars going supernova.
Blackholes
I could go on forever...

But that would kinda dilute the argument. With this kind of inclusion, every single game WILL be different in so many more ways than how it is at present.