Travis

He did indeed use it. Remember the fight with Pikkon. Goku used it and he quoted, SUPER KAIOKEN!!! And you got the concept all wrong. It's not a transformation. It is a catalyst for power. This means it uses whatever energy you have and multiplies it, thus giving you enhanced abilities and power, but at the same time, taxing your strength that can take a toll on your body if used excessivell. The question is, what are the limitations. He used Super Kaioken. How far can this technique go? The reason why you belive he don't use it is by theory, the writers of the show may had thought that it is best to keep this arsenal to a limit so it dont comlpicte the story. They most likely limited its use because of what it can do in times of DBZ. Here is another thought... Cell could had easily defeated Gohan. Cell had Gokus cells remember.