It's more about how you use the spells, for example the lower level turn undead spells become useless at higher levels according to some people, but since they will always, and I mean always cause an undead to stagger they can be used to open up opportunities to attack much stronger undead without using a large amount of magicka.
The same can be said for other spells.
Place a rune in a doorway and it will be activated by the first person to come running through, how is that not useful? Combine them with sneak and the quite casting perk and you can be placing them under the feet of bandits without having to worry about being seen.
Blizzard can be used with a little help from Become ethereal or depending on the character sneak or a well timed invisibility potion, it's the same for other master level spells. And if I remember correctly you can get Dragonskin to last for a couple of minutes with the right perks.
I think the major thing to think about is that you can boost any spell in Skyrim to be more successful or use it in a way that makes it successful.
Conjure Familiar can be boosted with positive poisons, hit with and Ash Shell and become an immortal wall for 30 seconds (although that can be boosted)
Runes can be used to deal fairly decent damage to enemies that walk into them and can be set before the battle even starts, let your Magicka regenerate and then have a bomb ready to go off.
Ash Shell and Rune can be used with Fury to create an immortal punching bad while you fire Poison Runes or Ignite Spells into the enemies that can still attack, dealing more damage and keeping you safe.
Blizzard can be pumped up to high damage but also slows enemies, sadly it does have that bug so that it is weakened by your Frost Resistance but oh well.
Transmute can earn you a fair amount of gold when you consider that 1 Iron = 1 Silver = 1 Gold... Iron is ridiculously cheap and easy to come by so transmuting that into Gold early on can mean a lot of Gold and it raises Smithing faster then any early-level Smithing.
Guardian Circle can be combined with the Atronach Stone to heal your Magicka up pretty damn fast...Really good for Mages with High Magicka and low Regen.
High Tier Illusion Spells have that handy area of effect ability, quite powerful when a small horde of Bandits. I mean from just the Rage and Kindred Mage perks you can cast Frenzy at all level 47 human enemies..
Pretty much any spell can be combined with other's or used in interesting ways to become quite powerful in its own right. Just gotta be creative.
My first playthrough was set in adept I think because,well it was my first playthrough. After that I raised it to master and from that point onward I never decreased it. However it's true that few of my characters went past Lv 30 and none was a mage. I like to set a small story like become the Archmage,do this good deed and that good deed which is related to a mage something and Archmage would probably do. But after a while I run out of ideas. So I delete that character and start a new one.
p.s. I never fast travel so it lasts longer and I also love the random encounters.