If you have a 200 griffin stack with stone skin/magic spring and bless that is attacked by enemies... well then you don´t need a fireball any more
Later in my mage game I really used tons of direct damage inflicting spells (fire rain, lightning, pole axes and so on), but those "troop enhancement" spells can deal (much) more damage in certain situations so I used them quiet often.
And if you are not a mage, low mana can prevent you from firing one fireball each round.