Humans are innately musical. While I agree that certain individuals excel at certain aspects more than others, its mostly when comparing types of music/musicians based on certain benchmarks that it seems so exteme.
Don't get me wrong, obviously people do have natural strengths in areas that greatly exceed others. Unfortunately, many people become disheartened by this and never discover their own unique potential.
Alot of it has to do with having enough confidence in ones own voice/gifts to develop it rather than imitate others. There are many master/pro musicians who are extremely well received/liked that would be consider down right awful in comparison to some else who has developed a different set of qualities. Think Tom Waits versus Parvotti.
I remember Carlos Santana talking about playing on tour with John Mclaughlin, he said that after John would play, at first he felt like, wow, what I am going to do after that?? But after a while he realized that his voice was unique and as shredding as John was, he couldn't begin to offer anything that Carlos had to give.
I think its really important for musicians to keep that in mind. Work to develop your sound, your own phrasing, tone, etc... quirks and all, based around your strengths. As long as you are speaking with your own authentic voice, there is no competition, really...
I personally started off as a drummer, and always had a strong sense
of rhythm. I also loved exotic (mid-eastern, harmonic minor, etc...) scales when I started on guitar. This greatly influenced my voice on the guitar. It took along time before I even became interested in the blues at all. I know alot of more well rounded players, but alot of them sound very similar to me, and I am happy to have "my own" voice so to speak...
As far as music study developing intelligence in children, along side of the reasons listed above, alot of it has to do with the way playing an instrument encourages simultaneous use of the right and left sides of the brain. If you google brain gym you will find there is a whole area of study (educational kinesiology?) that focus on these aspects of brain development.