what you arent incorporating is the variation as one ages. It peaks as you get towards your prime and then falls as you go over the hill. Most of the time, the best minor league IsoP numbers come right before coming up to the majors. In Melky's case, he was rushed due to necessity and his final season, the one that would make your IsoP case prudent, if it were in the MiLB, you would have your point. Take a look at a few of these players.
Carlos Beltran's IsoP
1995 .054
1996 .083
1997 .082
1998 .073
1999 .044 first season in the majors
2000 .062
2001 .056
2002 .073
2003 .082
2004 .100
2005 .064
2006 .113
look at the fluctuations in the minors, from .050 to .080 in one season. What would happen if he was in the majors after that first season? Obviously, Cabrera has a little more minor league experience than one season, but the fluctuations above show that both of us could be right, depending on where in the career you want to sample.
Here is another guy
Felipe Lopez
1998 .018
1999 .074
2000 .046
2001 .057
2002 .076
2003 .080
2004 .061
2005 .061 first full yr in the majors
2006 .085
look at these numbers. Lets assume that he was called up after 2001 and that was his IsoP? We would be in the exact same boat as Cabrera was in. My point is, the guy was called up too early. He came up and had an up to this point career IsoP year. If this .080 IsoP year was in AAA, we would all be talking about a guy who is an up and comer.