It wasn't really a 15-zhang LDA, but it's just easier to say that then to balb out the entire sequence of events starting at 15 zhang away and ending 3 zhang away.
Anyway, CC once said that it was a bad idea to launch such an assault because what if Ding Chunqiu used Ah Zi to block? Then Ah Zi would be dead.
But after some thinking, I think it would've been extremely unlike for Ding Chunqiu to exercise such a defense. Because Xiao Feng's palm attack was so unblockable, if Ding used Ah Zi to block, both he and Ah Zi would have perished. Since Ding doesn't want to die yet, it would've been better for him to leap back.
Of course, he could have leaped back with Ah Zi and THEN throw her to the front. But by then, most of the energies would have been dispersed by the air already, so no harm could then be dealt on her.
So given the circumstances and the possible outcomes, I think it was a relatively safe decision to fire such an aggressive assault.