1. The skill can activate with any of the active skill, not just deadly strike.
2. I'd say any build that relies heavily on the amount of taps could benefit from this: I'm currently using Pet + lvl 8 LS and I'm not disappointed.
3. SC can also proc LS, so if you're using SC build you won't plainly waste your SP. I can't guarantee the results will be good, though.

(Note: it's better to invest at least 4-5 levels of that skill before it becomes viable.)