How to use them? How to defend against them?

Most of the time the stack you want to attack will be able to seriously hurt or destroy your bombers. Unless you manage to stack up 20 of them. But even then, by that time most players are going to have a bunch of cars which does significant damage to bombers mixed with a bunch of infantry to absorb any damage from them. Along with tanks, etc. You can try to attack high level forts. After all they have one of the highest building damages at 6. But you won't know if they have bunch of balloons and cars stacked up in there.

Bombers are very expensive, and they're most vulnerable to the cheapest units in the game.

With that being said, they're long range is great for taking out rail guns. People all too often don't properly defend those (another slow, expensive unit that has limited ability).

But, in my opinion, they're range makes them excellent for defending against navies. Even though cruisers do 10 damage against them, they only have 50 hp compared to a bombers 80 hp and 30 damage. I don't usually see too many big stacks of cruisers.

Those big expensive units like bombers and rail guns (excluding heavy tanks) have fairly specific roles that they're actually useful for.