Dead Thralls are interesting for variety's sake, but after some experimentation, I prefer the Wrathman spell (you can find the Tome in the Soul Cairn via the Dawnguard expansion). In conjunction with the Necromancy, Dark Souls, and Twin Souls Conjuration perks, you can summon two Wrathmen with 900 hit points each for a duration of 1.5 minutes. Although the duration is not permanent, I have found that they hang around long enough to help finish off most encounters on Master difficulty before they expire.
Wrathman is an Expert level spell, which means you can obtain it at an earlier level than the Master spell Dead Thrall. The casting time is also much quicker should you get in a pinch and need aid immediately. Master level spells consume a lot of Magicka, but they also take a while to cast, which in my opinion makes them less efficient. Also, with the Wrathman spell, you don't need a corpse like you do with Dead Thrall.
The Wrathman doesn't do a lot of damage, but he is one very tough customer, and will keep your enemies busy long enough for you to raise either your Magical or Weapon skills in the process of finishing them off. Also, Dead Thralls appear to get lost after a while if you run too fast or too far ahead of them through complex terrain, which I find annoying when I suddenly become surrounded by enemies.
So while technically not the most powerful or highest level spell, Wrathman does have its merits.