I put the Hammer in a lot of non-Mind books and would never consider Piercing Thunderstrike when they cost the same sbp... so it's really hard to turn down the Hammer when it's in school
I agree it's an unconventional move but the situations we're talking about here are different imo.
I also use FH frequently in my books but they are meant then as a one-use countermeasure to some situations that arise.
Each copy will be cast just once and then be lost.
In this case however the spell is meant to be stuck on a Spore so it can be cast every round.
I'm sure we agree that 9 mana for 6 dice ( most of the time ) every round doesn't sound very viable.
As an example : consider a ballista as target. 8 life 2 armor
Even with 8 dice the Force Hammer has less than 50% chance of taking it out in 1 shot.
You'll likely need two shots making this a very expensive deal at 18 mana.
Piercing Thunderstrike will do 4 damage on average and so will on average take it out with 12 mana.
So my point here is, Force Hammer would still be the better choice if it can eliminate it's target in 1 round on average , so anything with a combined 8 Life/armor total.
Piercing Strike's main job however is to attack creatures, many of them having 1-2 armor and that's where the damage output is the same, only the Hamer costs 3 mana more.
Against creatures with 3+ armor Force Hammer becomes better again ...
We can't have it all ... choices must be made