1.Cous I,You and others like fighting scenes not One shot Fighting scenes.....
2.It's stupid to use Kamui right away and give enemy Info about your fighting style and give info about your Strongest attack.
3.If Kakashi wuld kamued Full body of yahiko He wuld be tired to fight all other oponents and wuld lose fight advantage(by not Understanding how rinegun works and how to fight or trick that kind of oponent with visual powers.
4.Kakashi is not only Kamui thing he has much more in his arsenal and uses kamui only on strong oponents(He wants to Trap his opponent with strategy first and preserv as much Chakra as pousible by using jutsus,Taijutsu or genjutsu).