Just not enough utility to use in any deck that I can think of.
It... drops their damage... to one third of its original value...
Sure it does. Nothing I'd ever use though. The utility is pretty poor. The most dangerous creatures in the game are things like Otys, Devourers, Mind Flayers, Nymphs, Firefly Queens, etc... Of those dangerous creatues this card is about worthless. And totally worthless against immortals. Life is set to stomp opponents into the ground with a fast damage output. This card at best counters a bit of an opponent's damage output, which in most matchups if they are trying to simply outdamage you, they are going to lose anyway.
Maybe you'd use it, but, I surely wouldn't. It would just slow down my deck, and that would hurt me more than my opponent.
Otys: Sure, not the best target for this
Devs: ^
Flayers ^
Nymphs: The majority have a relatively high attack, making this quite useful for nymphs.
FFQ: Underused, too easily blocked by shields. Not as dangerous as you think it is due to its speed.
"totally worthless against immortals" - no offense, but this line just makes me think you're stupid. I don't see why you're using it as an argument point when it has no relevance - because every other spell besides SoW also doesn't effect immortals.
" This card at best counters a bit of an opponent's damage output" - I don't think you see just how powerful this is when combined with shields. First of all, the attack is divided by 3. Take the biggest attack creature, Ruby Dragon, for example. At 15 attack, this card reduces it to 5 damage per turn over 3 turns. Then add Jade Shield (used in this example for being in-element). You've now reduced a huge dragon to 3 damage a turn. Three. damage. per. turn. If this example doesn't show you just how good this card can be, then I don't know what will.
"which in most matchups if they are trying to simply outdamage you" - I have no idea where you're getting this context from. Sure, there are times when there are more creatures than you have control for, but that's ONE TYPE of matchup. There are so many others in which this is useful - more in generic use than specific matchups.
"It would just slow down my deck", "Life is set to stomp ... fast damage output." - You seem to be forgetting the stalling parts of life - Heal, Empathetic Bond, etc. If this would slow your deck down, why put it in? Not all life decks are 6 frogs 6 cocks 6 adrenaline 12 pillars. This is a great card for stall decks, the reason of which should be obvious.