They have fantastic range, deal decent damage to soft targets, and absolutely wreck enemy cities, which yields fantastic long term results. Their range makes them great at scouting, even with that poor line of sight, and also allows you to send bombing raids deep into enemy lines to cut off enemy reinforcements, or prevent those reinforcements from being produced in the first place. They certainly aren't for every situation, but they are by no means a bad unit.

That's why you don't use exclusively a single type of unit. I'll send some strats out, and when the enemy gets wise on that, I'll send in fighters (if I have enough) and use their change in production strategies to launch another push on land, forcing the enemy to produce more anti-ground units, allowing me to fight enemy fighters and send in the bombers, and so on, and so on...
Yep, diversity is key. Too much of one unit, any unit, will end up in your destruction against a good player.