People are souls that inhabit and animate human bodies. The lifeforce of an individual is what powers the soul and allows it to excercise control over the body. Physical and mental attributes, such as strength and intelligence, health, etc, are due to the attributes of the soul which are reflected in the body.
An ardat yakushi is a human that needs to feed off the life force of individuals by sucking out the soul and consuming it in order to survive. By this method, it increases it's own abilities, and gains the attributes of its victims. With every soul it consumes, it gets stronger and smarter, gaining the powers of the unfortunate person it had eaten.
However, there is a problem with this ability. By inheriting the positive traits of the person who's soul it had eaten, it stands to reason that it would gain all the negative traits as well. Diseases, handicaps, or other deficiencies would be transfered from the victim to the ardat yakushi, leaving it with those same defects. As all human beings are suffering from some issue, some even unknown to them, this would create problems for any opportunistic soul sucker and make feasting on other souls less than worth it.
What would allow this creature to use its powers of eating souls without suffering the negative drawbacks?