Beyond the fact that such advice makes me wonder whether we really are as missionary-oriented a religion as we tend to think, i suspect that such advice actually stems from people porting in ideas from
Consider: If the number of youth who are Mormon nearby is larger, it’s easier to have Mormon dates/friends who are good influences selected from the total pool. If you’re a youth who lives near three other Mormon youths and two of them are, um, not quite living according to the principles of the gospel, your good-influence options for friendships among Mormons are terribly restricted. If, however, you can choose from thirty nearby Mormons, even given the same ratios so that twenty of them would be bad influences, that still gives you ten good-influence Mormons to hang out with.
(Of course, this leaves out the fact that the sort of clannishness that advice offers isn’t healthy at all, and also that one has to wonder who’s supposed to be a good influence on people who aren’t themselves good-influence types, but i’m guessing anybody who regularly reads this blog either already knows that or already loathes me enough that they wouldn’t believe me on such points anyway.)