It's a valid point. But having seen many examples (both success and failure) I don't think that the notion of a mission changing a person for the better is invalid either.
Though it's not a case of p-rn or fornication, consider one of my experiences as a missionary in the mid 1980's. When I was a ZL, we visited a companionship around lunchtime. They were still in bed undressed, unshowered, and playing Nintendo. They each had two months till the end of their service and of course were very embarrased.
Neither of them had had any success (baptisms they could call their own) so far on their missions. We noted that time was short and wouldn't it be great if when they gave their homecoming talk, they could at least be partly honest about it? We also promised them that if they obeyed the rules ungrudgingly until the end, the Lord would bless them with success.
We did check up on them periodically after that but they seemed to have made the change. The week before they went home, they had two baptisms.
So while there were many failures, there were also some successes. I worry that because of fear of failure, nothing gets accomplished and no one's life gets changed. So I worry a lot about what I see as a general trend in the Church to withdraw from possible conflict and become more pharisaical and puritanical and in other ways, more appeasing of evil.
BYU's Student Life and the way the honor code is enforced is another example of this In my humble opinion. At some point, we have got to trust and believe in the principles the Church teaches such as train up a child in teh way he should go and when he is old he will not depart from it.
Socialism and the welfare state is yet another example of this to which I address on other threads. We are becoming a people who believe that "charity" must be enforced on others. We don't trust and therefore we don't change and don't have real charity and instead have become thieves and robbers.
Edited: bcspace on 20th Jul, 2011 - 7:42pm
The church's mission presidents say something like that, not all of them but some of them do that are baptism and numbers hungry. Nothing wrong with bringing in souls but it shouldn't be a contest.