Firespray isn't amazing, but I wouldn't call it border-line useless.
Boba and Kath can both be flown at PS9. Both of them- when flown properly- get a 4 die primary. And if you are flying Boba properly, you also get a free Predator.
Boba- because of his ability and rear arc- can be built to tot care about stress. That means adding Zuckuss will be _very_ painful for your opponent.
Kath? She prefers to fire out the rear arc. She _has_ a rear arc. You know what can use a rear arc as well as a primary arc? Tactician.
And, of course, both pilots love Glitterstim.
Borderline useless? Not even a bit.