Outside of meta, harvest life does relatively average damage + generates fury. Using HL while in meta does a fair bit more, but generates no extra fury.
The bug here is casting HL in meta and dropping meta afterwords, you'll gain both the damage benefit from meta + the fury generation from caster form.
In heroic ToT, helpful on meg adds, primordious, twins, and MAYBE lei shen ball lightning if you messed up on saving fury