I seriously doubt IS by itself makes you good. My starting C
(11435847), though averaging the 2nd most minutes on my team, is also 8th in scoring. He is respectable IS, and atrocious JS. On the other hand, jump shot and little inside scoring doesn't help much either, as my backup C
(10592590) has inept IS and average JS, and is 11th in scoring average (though he does have more points per minute than my starter (9.4 per 48 min vs. 11.7 per 48 min))
What I'm thinking is that IS makes you good at certain shots, while JS makes you good at others. I would imagine that IS would apply mainly to when you are posted up, while JS would apply when you get the ball facing the basket. This makes sense based on my experiences with basketball (i.e. I suck at both, but for different reasons). I would guess that running a different offense can minimize their liabilities with one skill or the other (inside focus vs. outside focus).