So if the only people who go to hell (according to Mormonism) are those who joined the faith and then left it. So wouldn't it be better to not try and get more followers? Wouldn't it be better to have people not join the religion than risk them leaving at some point in the future and ending up in the Outer Darkness?

Yup. 

I think that they originally created the idea of Outer Darkness as a means to keep members from leaving.  (Only people who leave go to hell!  You don’t want to go to hell, do you?!) 

The problem is that a very large portion of Mormon doctrine is kept from potential members until after they join.  In fact, a large portion of Mormon doctrine is kept from members, period.  There were things I didn’t find out about the church until after I had left.