The Tavern

I have never used it, but it seems mediocre. Simulacra do tons and tons of damage, but are very fragile. What makes the spell good?

Their fragility doesn't really matter. Most monsters can only attack one thing at a time, and one casting of Simulacrum makes "a bunch" of simulacra. If each one does half of a monster's health in damage, and you make 5 of them, then a turn goes something like "The monster attacks one and kills it, 4 of them each do half of the enemy's health in damage, you win the fight." They (like all summons) keep enemies distracted from attacking you, AND they leave clouds behind, making them excellent at keeping enemies away from hitting your fragile body. Couple that with the fact that monsters would rather attack you than the simulacra, and you'll find that losing your simulacra isn't actually a huge concern most of the time.

Yeah — they're more like excellent, weird conjurations than they are like Yred allies.

Also, in particular with Kiku's corpse delivery, you can make a really large number of very high-damage simulacra.