The secret art of fusing with Ancient Divine Beasts might harm the beasts. That was only Ling Tian and the others' analysis and speculation; they weren't certain. If the secret art didn't harm them, naturally they'd be eager to learn it. After all, it could bring a qualitative leap in strength. For instance, if Ling Tian fused with Xiao Shi now, he could probably reach the peak of Quasi-Sage sixth rank with ease, or even Quasi-Sage seventh rank. At that level, they could easily kill every one of Chi Xue's people and the Beast Taming Cultivators—perhaps without Fairy Meng Shang and the others having to help at all.
If Ling Tian and Xiao Shi broke through to Quasi-Sage eighth or ninth rank before fusing, their strength would be even more terrifying. They might genuinely break through to Saint rank; at the very least, breaking free of the Lord of the Universe's constraints would become much easier.
Even if the secret art of fusing with Ancient Divine Beasts did them great harm, it wouldn't matter much. Ling Tian, Old Man Ling, and the others excelled at modifying cultivation techniques and secret arts. Perhaps they could alter the original art into one that didn't harm the Ancient Divine Beasts. Then Ling Tian and the others could learn it, and everything that followed would become much simpler.
Moreover, once they obtained the secret art, Ling Tian and the others might notice its weaknesses and use them against Chi Xue's people and the Beast Taming Cultivators. At the very least, it might let them draw useful lessons from it, since many secret arts shared similarities.
In short, there was no harm in studying the secret art. That was why Fairy Sword Ji and the others urged Ling Tian to capture one or two Beast