I'd have to say in games that both sides are developed the carrier pays for itself. If the AI used them they would be much more nastier, but they come in handy for two scanerios I run into commonly...
1. If I don't have 'jump cities' to transport my nuclear missles within range of the enemy nothing is more helpful than bringing a carrier full of them close to your enemy's cities.
2. When civilizations are par technologically in the end and you want to establish a strong invasion and avoid nuclear weapons. You can transport a legion of bombers and fighters to get rid of any bombers and empty out a port city to give you a safe landing spot.
Often I'll send one settler over (sometimes before the conflict) to make a fortress.
1. If I don't have 'jump cities' to transport my nuclear missles within range of the enemy nothing is more helpful than bringing a carrier full of them close to your enemy's cities.
2. When civilizations are par technologically in the end and you want to establish a strong invasion and avoid nuclear weapons. You can transport a legion of bombers and fighters to get rid of any bombers and empty out a port city to give you a safe landing spot.
Often I'll send one settler over (sometimes before the conflict) to make a fortress.