Here's what I find interesting...
As Miz pointed out, JWs don't use Mark 9:38-41 very often. However, they do pick up with vs. 42: “If anyone causes one of these little ones—those who believe in me—to stumble, it would be better for them if a large millstone were hung around their neck and they were thrown into the sea." This is often used to keep people from going ahead with conscience matters like dress and grooming, entertainment, recreation, etc. These are matters where the choice is not outlawed by The Society but could "stumble someone." So, OK. It could be used that way. But when read in context, it really applies to the circumstance mentioned in the previous verses.
To ignore the primary message of the scripture (don't stop anyone who is trying to promote Jesus in a Christian manner because if you do, you are both hurting the chances of more people becoming Christians and stumbling someone who is trying to be Christian) by acting directly against it ("all other so-called Christian churches are horrible and will be destroyed by the very being they profess to worship") is ridiculous.