A bit back to topic, to sum it up:
1. Unit stacking allowed gathering very variable power on a single tile. In this case city strength didn't work, so instead cities provided bonuses for defenders. Also, with unit stacking it was possible to make units cheap, so having a unit in each city wasn't a problem.
2. In 1UPT units are more expensive and require more turns to build, so hard requirement to have defender in each city is a problem. Also, with limited number of units which could attack a city, it's easy to calculate the required city strength for each epoch.
This means for stacks it's natural for cities to provide defensive bonus, while with 1UPT it's more natural for cities to have their own strength. The Civ5 problem was not the city strength itself, but crazy ranged attack.