The thing is, Kojima was right (as was Miyata), Ippo is pretty predictable. He is great at what he does, and has a lot of strengths that make up for it, but he is repetitive, simple, straight forward, etc. In other words, his training is a blessing and a curse. It would have let him down.
The Coach (and the author) are so wrong in this instance, trying to feed Ippo (and the readers) this idea that it was a bad thing Ippo mixed things up. He will never make it on the world stage following his training like a robot.