Well, when I came up with that Fireball I tried to make its utility somewhat limited, Unlike a firearrow, the caster has to physically throw the ball of exploding fire, making it roughly equivalent to a magical grenade. And like a grenade, once it leaves your hand Mr. Fireball is no longer your friend. If you drop it, it will go off at your feet and YOU will take the full force of the damage.
Clearly this is not a spell for when subtlety is required, as the loud bang it makes and its tendency to ignite things will garner attention.
Speaking of subtlety: is there a rune spell conceivable that would enable one to make one's movements silent, or to make the weapons one is carrying less noticeable?