Yeah, TA/DA spying needs to be improved. The AI doesn't appear to use spies, ever, except to create a massive counterespionage reserve, which they use the instant you put a spy down. Spying simply becomes an economic race - you have to produce spies faster than the AI can create them. If the AI used spies it might be a little better but... the fact that they don't is a little crap.
The doubling returns on purchasing spies is a little crap too, as it means you'll eventually just run out of money for spies as they cost you 1,593BC a week to get a spy in a month. At first it seemed a little cool, but now that I found out getting any intelligence on any races after the beginning of the game ends and the midgame begins is nearly impossible and costs trillions of credits, I lost interest in espionage.