Why a changling and why the soul grinder
Good questions, I used the changeling for contesting my opponents backfield objectives, use scanners, etc., and gives an option outside of nurglings to accomplish this... Soulgrinder, currently is untested but I tried two defilers last game and they were surprisingly good (not omg good but still very good), the list had 10 left over points so I dropped it into the grinder so I could gather some more data. If it performs as well as the defilers did, I imagine it will find a way into other lists.
The defiler gave a good melee and range threat that required more than just chaff thrown at it, and it could cause concern for both infantry and vehicles.