The spell is most often used between a soul entities such as a lich. When a lich encounters a lich or other undead creature that has managed to overcome death like a possessor that moves from host to host this spell is used to make them effectively insane and useless. It is alot easier to render somebody else a non-threat in person than it is to find where ever their phylactery, that would be guarded. Also not all methods of soul preservation would be the same so if you encountered something similar it could take a long time to figure out how it animates and maintains itself. The simple solution is to just to neutralize the threat as opposed to its life. The purpose of a soul meld simplified, its to kill others like you for self preservation=d> .