The choice to use the Genkidama was obviously out of desperation, which logicaly, if there desperate to save there lives and there loved ones, bringing Gohan in would of been the smart thing to do. But Toriyama couldn't think of a way of getting Gohan involved and still keep Goku front and center.
Personally I would of made pure Buu stronger and have Gohan keep him busy while both Goku and Vegeta make there own Genki Dama (Vegeta comes up with the idea revealing that he learned the technique).
Gohan beats Buu down, throwing the weakened Buu between Goku and Vegeta, who then throw the attack, hitting Buu from both sides. The two Genkidamas then merge into one huge one, and explode, taking Buu with it.
But then doing that would of gave the win to Goku, Gohan, AND Vegeta, and we can't have that.


