Which NPC does this? Also, nothing in Bind Soul implies that you could move your soul into an empty body. It does say it's open to possession by a Restless Spirit, but I'm pretty sure that's specifically undead, isn't it?
However, if it wasn't, you still leave yourself open to instant destruction by a Dispel Magic spell of PL3+, so it's immortality at a risk.
It is a blood mage in the Pelagia adventure site. A body is a valid vessel if it doesn't contain a soul. This is how Ravil (the blood mage in question) does it. He removes the soul from a victim and by that creates a valid vessel. He then casts the spell again moving his own soul to this vessel. His own body will be unconscious during this time. Since there is an NPC in the official campaign who do this (to achieve his goal), this is one way to use the spell. Ravil risks a whole lot by doing this, yes, but he clearly thinks that it is worth it since he will stop at nothing to achieve his goal.