I think people are really getting stuck on the numbers here... the max amount of fireballs you can send is 10. That means the max fireball hit is 160k it costs 6k in sapphires to purchase 160k damage. As somebody who spends money on this game, I'm not sending 6k sapphires to blow up your castle unless you really ticked me off, in which case you know its coming. If you think a coiner is going to waste a sapphire on your piddly little castle you are wrong.
Fireballs are really just a strategy item... they don't deal any REAL damage so they're really not valuable for warfare. The fact that they are unphased by castle walls and travel fast is literally the only worth they really have. You guys don't realize that fireballs can be used to even the odds against a stronger opponent. Guy has a super buff wall? Have some fireballs land before your troops do, it'll help your ratio. Have somebody marching back to their castle? Try to land a fireball hit that lands right after his troops, you'll likely destroy some offense, and if they don't have any defense out you'll pulverized their offense... they MIGHT be a way to get non spenders to put money into the game, but they don't really help the coiners out any... in fact they only make coiners weaker.
Thank you for your feedback, my Lord!