Figuring that the draft was started to use colleges as testing and proving grounds for players, I have to assume that for baseball the experiment has failed.
Many feel that the introduction of the aluminum bat cancelled all bets.
Today where a college education is almost mandatory to become successful the only real reasons to sign a pro contract out of high school would be reasonable compensation or exceptional skill levels. The idea of playing out a dream is also a good reason though not usually one that is profitable.
I think that more often than not scouts regard potential far too high as compared with existing skill level.
The other side of the question is time and age. If the average pro player takes five years to reach MLB skill levels, that player would be four years older than the kid signed out of high school or a foreign country most often.
The college player has a better foundation for the rest of his life than a high school signee. The high school signee has the advantage of applying those same five years at an earlier age.
Skill level plus potential, in that order, should be the determining factor. It is a better starting point than having more potential than skill.