I find the current characteristics of siege weapons in Civ 5 Gods + Kings to be well balanced and very playable. You can make strong cities to go tall, or swarm units to go wide.
If the developers nerfed the catapult/cannon/etc with the buffs you suggest, it would be too hard for a tall-strategy player to defend their cities. The answer you are looking for as the attacker is multiple units. If you move a melee, a range, and a siege unit into range of a defending city, you will find the AI (and smart humans too, on multiplayer) will target the melee unit, not the siege. After all, the melee unit is the only one that can take the city.
Your goal as city defender:
eliminate melee units before siege units do too much damage. Your city fire, plus range unit(s) should all fire on the same incoming melee unit - dont let them get heal promotion
Your goal as city attacker:
get your melee units adjacent to the city, preferably in rough terrain, to attack the city in unison with your siege.
In general, siege is a complement to your primary attacking force - melee units. Perhaps too many players try the reverse - level the city with siege and then waltz in one melee unit to take it when reduced to zero. This strategy is slower, harder, and more expensive. Be willing to take casualties. Indeed, I would say siege is not an absolute requirement to take a Walled city. They don't become a must-have until Castles come along.
If the developers nerfed the catapult/cannon/etc with the buffs you suggest, it would be too hard for a tall-strategy player to defend their cities. The answer you are looking for as the attacker is multiple units. If you move a melee, a range, and a siege unit into range of a defending city, you will find the AI (and smart humans too, on multiplayer) will target the melee unit, not the siege. After all, the melee unit is the only one that can take the city.
Your goal as city defender:
eliminate melee units before siege units do too much damage. Your city fire, plus range unit(s) should all fire on the same incoming melee unit - dont let them get heal promotion
Your goal as city attacker:
get your melee units adjacent to the city, preferably in rough terrain, to attack the city in unison with your siege.
In general, siege is a complement to your primary attacking force - melee units. Perhaps too many players try the reverse - level the city with siege and then waltz in one melee unit to take it when reduced to zero. This strategy is slower, harder, and more expensive. Be willing to take casualties. Indeed, I would say siege is not an absolute requirement to take a Walled city. They don't become a must-have until Castles come along.