The MLB scholarship plan is a very shrewd PR move. While it is a fine contribution to a kid's college fund, it by no means comes even close to paying for the cost of a college degree.
First, the money is taxable as income.
Second, by the time most HS players can use it (let's say after their initial contract expires and they are released, about five years) the cost of tuition has experienced five years of inflation, while the amount under the MLB program remains the original amount. (If a kid remains in the minors for even more and never reaches MLB, the inflation bite is even greater.)
Third, the room and board portion of the money is based upon the cheapest housing option offered at a school. So, the 24 year old player is now rooming in a quad with a bunch of 18 year olds away from home for the first time.
Fourth, while delaying college reduces the chances of finishing college for many, for some (like me and many others) it is a viable option. The trouble is, one isn't able to predict what life deals which may make attending college extremely difficult (wife, kids, perhaps)
Fifth, if a kid was accepted to an extremely high academic school, his slot will be gone. For example, Princeton will hold a slot in their class for only two years if a kid wants to detour first to pro-ball.
Sixth, as a kid moves from high A to AA, he gets a modest bonus. That bonus comes from the MLB scholarship money and reduces the amount MLB owes. It's not a lot of money, but every bit counts as every year in the minors the kid is operating at a cash flow deficit (meaning either a significant parental subsidy or chewing through that bonus money).
Seventh, the money must be used within a specific time period; that time period may be too short of a kid has begun a family and must attend college part-time because he needs to maintain a full time job to support his obligations.
(I believe the info above was correct as of the last CB agreement; if the info has changed, please correct it. I don't keep up anymore on that fine print.)
The devil is in the details. And one problem is that there are very few truly dispassionate people to weigh all the issues; all opinions received must be viewed with an eye towards that.
And none of this missive really even touches on the life that an 18 year old has for the first 2-3 years in proball. It's only nice and romantic from the outside looking in; it's a brutal business from within.