Also Fandangalangadingdong didnt have the power specifically by default to pull another's soul from their body, he was actually using Alus's souljacking technology that Fandaniel recovered from Alus's lab in Ala Mhigo (you know, the same thing that was used against you in the second boss fight of Ala Mhigo where you're pulled out of your body and have to get back to it quickly or else you're KO'd?). to pull the WoL's soul from their body and put it into the corpse of a deceased Garlean centurion (it was strongly presumed the corpse was the same one Zenos used temporarily until he got his own body back during SB's story). This is why your soul ultimately is ejected from the host body, it's the limitations of Alus's technology, and it was simply a means to an end. Like much of EW, it was a callback to an earlier plotpoint in the story.

Fannyboy and Zenos wanted to see just how powerful you really were, and the actual source of said power, that maybe it was your body that was the key (hence why Zenos then takes your body for a joyride to Camp Broken Glass while Fandaniel drops you in your borrowed Garlean meatsuit into the ruins of Garlemald.

But it showed that it was your soul as well that made you who you were, given you're the reincarnation of Azem the Wanderer, a more powerful Ancient than Fandaniel ever was, and putting the will to protect others and their needs before your own which is where your power truly lies. Something neither really understood (Well, Fandaniel did once, a long time ago, but that's another story.) Evil cannot understand good and all that.