It's pretty solid training tool versus challenging creatures since you can use it in addition to roars to double up on Debil effects. FWIW, I have ~720 weapons vs only about 600 debilitation and I have no issues hitting at-level creatures with it.

Pros:
- AOE cyclic effect.
- Runs off Inner Fire rather than Voice Pool.
- Doesn't incur Roundtime.
- Starting the berserk trains Debilitation even outside of combat (which is probably not intended but hey).

Cons:
- Weapon range only.
- Pulse timing is not consistent.
- Not very effective vs. single targets or in PVP.
- Can be too effective vs. easy targets; ie. your defense learning can suffer if creatures are constantly being stunned/knocked down.

Personally I don't waste slots on earthquake as it has no pvp implementation, and debilitation can be trained perfectly fine with roars.

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Even given the painful slot limitations of magic tert, I'm comfortable keeping Earthquake berserk at 2 slots.

I already have all the PvP-centric stuff (anti-magic forms and meditations, Slash and Owl, Python/Piranha). I also spend a decent amount of time outside of combat doing forging, box popping, mining, etc. so it's useful to have an ability I can just pop for 5-6 mindstates of debilitation experience even when outside of combat.