Why is is bad? Doesn't it go along with the idea of securing your borders and all that jazz? It sure is helpful to be able to do it to the AI, so they should be able to do it to us as well. Only fair, right?
I have my own strategy for securing my borders.
I build my cities right ON the "rich" tiles, i.e. buffalo, wheat sheaf, gold etc.; this has the multiple benefit of ensuring a decent produciton of shields/food/trade at every turn, PLUS it means i will build cities more or less every three tiles.
What does this mean?
1) With roads, cities will be able to aid each other by sharing defensive units amongst themselves (i.e. I can have two Phalanx per city, and share cavalry); futhrermore, as my civilization grows, "inner" cities become safer and cavalry will be moved further towards the border;
2) With roads, it means that when I build caravans to help build my Wonders, these caravans will hop from one city to the next until they build, so they're safe from enemy or Barbarians;
3) When cities grow beyond size 7 or 8, they will eventually grow automatic entertainers because they run out of farmland; these entertainers I can then convert to scientists or taxmen at no expense;
4) When I build triremes, they will be able to ship goods directly from one city to the next, or use those ports as safehavens in case of enemy navy.
When planning to build a city next to an enemy city, I plan ahead:
1) I place one or two caravans so that my chosen spot is left vacant, and enemy units of any kind cannot move into it;
2) I place two or three engineers in the area, building a road from my ZOC to the chosen spot;
3) I keep two or three cavalry units in a waiting zone, ready to zip into the newly built city the moment it's created, so that by the time it's the enemy's turn he has MY city in HIS territory, with military units AND innocent helpless little caravans preventing him to take any action... snicker snicker snicker... it worls best after you've built the Great Wall, so you have instant city walls AND he must accept a peace settlement anyway... snicker snicker snicker... build infantry units and catapult asap and move those cavalry units backstage...