Attending college for 3,4,5 years working towards a degree before one is drafted (or in the game), IS having plan B.
I think that most people that come out of the game (over 24-25 let's say) are mature enough to understand that they need to think about life after baseball at some point. These discussions are more important for the young player in HS who actually has no clue what milb life is all about and the hard work it takes to get past A ball let alone to MLB.
I happen to agree with PG's post, he always seems to add reality to these types of discussions.
BTW, luck has a LOT to do with the ODDS. Like any job, it can be about being in the right place at the right time, the right team, etc.
Having said, that mine is 28 and still playing, not ready to stop until someone says he can't anymore, his odds are becoming less and less due to his age and he knows that. I am sure that with that in mind he has thought about life after playing the game.
It's not what you see on TV.
Here is a good example of why prior discussion can be beneficial. I remember son telling me about a player he roomed with (son was sent back to Jupiter to end his rookie short season due to sore arm) who was a late sign for 10,000 (before taxes). At the end of the GCL short season, he told son that his experience was nothing like he expected pro ball to be. Appears that he was very unhappy he had not gone to college. Obviously no one informed the 17,18 year old about what it can be like your first season coming out of HS. Signing for lots of money usually helps ease those feelings. The odds IMO were not that high for him as a late sign out of HS, and someone should have told him what options may have been better for him, or if they did he wasn't listening.