Once again, late round players are usually chosen as fillers, the organization will try to develop their talent enough to be what is called an "organizational guy".
I think that most parents (I am talking HS now) understand the odds against their player ever reaching AA, let alone ML. What a lot of them do is allow their son to follow their dream, I think that is a great idea, but a large bonus often helps the player more comfortable as he goes through the system and you and I know the more the organization pays out for bonus the more time the player has to stick with it. I hav ealways said here and people know it, if it's about the money go to college, if it's not, go pro.
Now every once in awhile you will get an exception to those very late picks, a good example is Tony Sipp (reliever for Cleveland). Sipp, a position player in college with a dynamic arm, was converted to pitcher by Kevin O'Sullivan (BTW, do you know him), drafted very late, but signed for I think 135 (give or take). The reason being as he was relatively new as a pitcher, and most teams were afraid, he did prove the Indians to have made a good choice.
I do resent the comment about the scout not doing his homework about the player who was my son's roommate, it is my beleif that scouts don't draft players unless the player has indicated he will sign, this one did. As late as he was, I would almost doubt the organization didn't spend much time digging up much as they weren't going to pay him much anyway. Perhaps he told him he'd take anything to go play, you don't know that, but you do understand that at the end of the day complex ball IS NOT what most players expect pro ball to be.
You might be stuck in what was vs what is, things are a lot different these days, IMO. Give me one very good example of a very late HS draft pick (last 3-5 years) who signed and is currently playing in ML.