frags as spammable is bad mostly because of the annoying cast time.
I love the sustain on it, i love the extra skill slot you get
I really hate the 0.8 cast time, it makes it feel real bad and when it procs it auto messes up your rhythm.

here is a parse with frags as a spammable (2 pet, frags, prey, fury)
compared to parse with crushing shock (2 pet, crushing, frags, prey)
it's a big dps loss and worse, it's annoying to bar swap or use a pot or synergies with.

The damage is equivalent with other spammables. The sustain is a nice twist, but hardly worth the cast time on it's own.

If they remove the cast time i would use this skill as spammable. If they don't, i will not.