Wow. All good stuff here, and I take elements of all of those in my own games. e.g. Iron Man rules. If I might rephrase my original question, it was "what compelling game-reason is there to play turtle style, other than the way you like to play the game". The short answer seems to be, there isn't one, really. The AI is too weak and/or does haveenough advantages to challenge a good human player. But some people, inclunding me, are just natural turtlers.

Partial exceptions to that answer can be made though if (1) you are waiting for late-game units and/or (2) discoveries, such as new world and/or new factions such as Mongol/Timurid invasions. To my mind though they are hardly compelling, and it would be very boring for a blitz player to wait 100 turns for the Mongols to show.