This, on the other hand, is very useful, especially when combined with Conjuration Dual Casting. As noted there, there’s a limited supply of dead bodies, and you’re likely to spend some of them making more dead bodies. Increasing the duration of your reanimation is therefore a much more worthy investment than doubling the duration of your instant-cast no-material-resource summons. A nice nugget for no-nonsense necromancers.
If you’d rather not invest in Turn Undead and/or Expel Daedra, this perk can be a worthwhile addition to your already-Soul-Stealing conjured weapons. It’s a bit limited in scope, though, since it does nothing about living opponents, such as dragons, giants, bandits, spiders, soldiers, et cetera. This means its effectiveness in a lot of situations is pretty much nil, but it can make things easier in a few key points for you.
What can be said? A must for any necromancer. Anything that gets more use out of each corpse you animate is vital to your ability to function. This is why you picked up Dual Casting and Necromancy, and it’s why you’re going to take this perk. If you’re not fully invested in Necromancy, it’s going to prove a lot less useful, so you really shouldn’t spend a late perk point on it in any other circumstance.
Increasing the overall power of your Atronachs by half is a massive boon for those inclined to elemental Conjuration, and may even be worth it for mere dabblers, despite requiring the uninspiring Atromancy perk to pick up. If you have Atronachs a lot of the time, you want this perk. If you don’t, you want Dark Souls. If you don’t have Atronachs or undead... why did you pick up the Conjuration perks again?
A massive improvement to standard Necromancy or Elemental Summoning, Twin Souls lets you have two conjured allies at once, allowing any Conjurer to double the effectiveness of their battle summons. This works equally for reanimation or atronachs- or for one of each if you prefer. This lets you make your traveling duo of yourself and one follower (you do have a follower also, right?) into a nice-sized adventuring party of four (five if you count the dog).