I gave some suggestions (and a link) above. Having your creature or a summon active can result in them being targeted first, or block an attack meant for you, etc. Leaving this until later may result in it being ok when you return, or at least you will have better armour and can deal damage faster.
Actually it goes the other way: sometimes when the fireball targets my necrosaur, I die instead of him, even if there's a wall in between me and her. "leaving this until later" - like what, forever? I would, if I didn't finally beat her through a lot of save scumming and teleporting away into the Tower.
