Grade B prospects have a good chance to enjoy successful careers. Some will develop into stars, some will not. Most end up spending several years in the majors, at the very least in a marginal role.

That grading scale is way too broad for the B’s. Marginal role players could be B players? Then again, so could some of them that develop into stars? So the B category encompasses everybody from Stan Javier to Seth Smith?

It doesn’t seem very predictive, to me. Like you’re just throwing a bunch of guys into a huge melting pot based on something… talent perhaps… but then giving them a huge range of possibilities. Since you state the A grades are surefire stars, which implies that they’ll have long major league careers as a regular player, then the other grades should have at least a similar predictive quality, shouldn’t they?

Maybe move the “maringal” guys more in the C range, since the reason why they remain marginal players is often for the reasons you cited in the C grade explanation.