Im beginning to think its a load of crap. I dont have stats to back it up, so accept this for what it is; a hunch. Everyone seems so afraid of pitching prospects because the whole injury risk. While Im not saying this risk isnt a big issue, I do however think it is somewhat mitigated by the fact that pitchers are easier to scout and project.
Seems to me its a lot easier to judge a pitchers stuff and control than it is a hitters swing because pitchers are throwing thousands of pitches a year, whereas a hitter obviously wont swing the bat nearly that many times. You have a much bigger sample size to go with when evaluating pitchers. Its also generally easier to see good pitching mechanics and a good pitch than it is to see a sweet swing. Basically, its a lot easier for pitchers to pass the eye test than it is for a hitter.
Theres also far more advanced stats for figuring out just how good a pitcher is (especially with the pitch f/x data, which is sick by the way). Hitters dont really have the advanced stats. ISO is good, but other than that theres not a whole lot to go by. Walk rates can be misleading as can K rates, so its tough to really get a good, solid feel as to how good a guys plate discipline and pitch recognition is. Its also tough to project plate discipline because recognizing a fastball is a lot easier to do when you dont really have to worry about secondary pitches (think guys in the low minors). Im not saying it cant be done, but it would seem to me that it would be harder to recognize a changeup than it would be to learn how to throw one.
Overall I would think (and its really not a revolutionary theory) that when injury cases are excluded, the bust rate for hitters would be a lot higher than that of pitchers. Now days though, teams are going through great lengths to protect the young arms, so I would expect the injury attrition rate of pitchers to go down somewhat. If that happens, I think the overall bust rates of hitters and pitchers to even out, making TINSTAAPP a useless nemonic.
Again, all just a theory, but I thought Id share.