Get the cost reduction. You're not constantly casting and recasting summon spells in the middle of a fight. I have up to the Expert perk for the Dremora just so I can cast one in each hand at the same time without using fortify enchants. Mind you I also have about 300 magicka because I totally didn't know about the -100% magicka cost reduction thing until I was like level 40.
lol you don't really get all magic at 100% reduction, its possible to doube enchant items and get closer to it, but its a game mechanic, i have super low destructive magic costs but i still use up my magic pool using other fields of magic like conjuring, alteration, illusion, so its not free magic, you're suppose to be able to reduce a field of magic down to zero, its not a gltich..you still need a magic pool to be able to use all the areas of magic in a battle, even with zero destructive magic cost, i'm always using alteration, illusion, conjuring and they all use up my magic still...so i make sure my magic pool without any clothing etc is over 400, just so i have enough for everything else in a battle.