Also not how I remembered it, although my experience is from Civ 3 and Alpha Centauri. There are a bunch of people who don't have this experience so I wonder what's going on.
You're talking to a lot of players who likely only played on Chieftan AI levels: where the enemy AI would rarely meet you in the field and often times only retreat to cities. Even then, they would rarely try to stack-kill the player. It was always possible, but since the easy-AI never attacked in such a manner, the mechanic was basically irrelevant on easy/Chieftan mode.
On more difficult AI levels (Prince or Emperor), the AI would meet you in the field, and you'd have to worry about stack kills. And also worry about cheating AIs who just had more resources than you...
Now, thinking of this further, i think I might misremember this because I've never let my stacks be attacked in the field.
Best tactics is to drive tanks via railroads up to the city (sometimes building them in the fly using a ton of settlers in one move) and attack it with 20+ tanks and then move in 10 mech infantry for defence, and I used fortresses extensively.
