I use all schools of magic very efficiently as a high elf with the atronach stone. I don't use the [insert magic school here] magicka reduction, I enchant to get as much magicka as i can. I have 700 right now.
A different means to the same end, and it is pretty easy to carry around two sets of equipment... especially when robes and hoods weigh about as much as a book.
Besides, that 700 points isn't going to do you much good when you get hit by that arrow.

You can entirely negate the cost of two whole schools of magicka, and carry an alternate ring, robe, hood, and necklace to negate the other two. You can even create a set that reduces the cost of everything, with perks included, to around 60-70 percent I believe.
Reduction of cost is simply the most efficient use of a Mage of all schools, or a single school, in my experience. Its been easier for me to cast a few free spells, then switch to one that actually costs something, than it has been to constantly chug magicka potions. I seriously doubt anyone is casting many spells without investing in reduction, even the perk trees are designed in this way.
Though this is probably worthy of a poll, or an entire thread.