Spearwall at fixed cost tests your ability to predict enemy actions and offers significant reward if you are correct.
Also it doesn’t have to hit anything to be useful – denying enemy some area can be good enough on it’s own.
It’s cost is also not that high, since you regenerate 15 fat per turn (or 20 with lungs). Effective cost per turn is 15/8/3. And since you shouldn’t seriously rely on spearwalls without spec past the very early game, it really is just 8/3.
Riposte already has hidden cost per activation – every enemy miss adds 2 fatigue to target. Making it cost more per trigger and less for activation makes more sense, because right now the only thing saving riposte from being completely unusable is the fact that AI doesn’t understand that simply skipping turn defeats riposte (that is in perfect situation for riposte user, where there are no other available targets).