Perhaps if Cao Changqing had nurtured this drought fiend himself, it might have caused Tang Luo a lot of trouble, but this was someone else's treasure. To control it, one would need to pour vast amounts of Spiritual Power into the golden disk, first to maintain the existence of the netherworld gate, and second, to issue commands.
While multitasking isn't much of an issue for warriors, there is a limit to their total amount of Spiritual Power. If you add Spiritual Power to the golden disk, naturally, you won't have any excess to merge into your Spirit Art.
So although this crescent sword qi looked astonishing, it was all flash and no substance. For Tang Luo, it was less impressive than the Buddhist artifacts transformed by the willpower in the Medicine Valley by Yankong, let alone fathom why someone would be so senseless as to challenge the elderly Xu Lao to a match.
