It's all about world-building. I agree they should've made Mifune stronger and have more than 1 fight but from what we hear he could defeat the majority of ninja....He's so fast and intense that 'ninja aren't even able to cast handsigns'
HANDSIGNS TAKE 5 SECONDS TOPS. WHAT A BEAST.
There should've been more coverage of them in the war and less fodder samurai wearing the gross armour. Okisuke and Mifune were really awesome.