I would hardly say espionage is "worthless". Being able to investigate cities at wartime is an excellent ability, and very reasonably priced.
Also, the way your military advisor can tell you the exact units they have in their military (I actually only discovered this feature the other day - so others might not know about it either) is *very* valuable information if you're planning an attack. I love having at least 2 tanks for every infantry unit they have before attacking, and having a spy in their capital lets me see if I can meet this ratio. Being able to see where they are in the space race is also very useful.
That said, certain espionage features, such as stealing of technologies or exposing an enemy mole do seem like a bit of a write-off; but I wouldn't say the entire espionage system is - having a spy in the enemy capital is very powerful indeed.