It'd just be more balanced in typing. Instead each starter has 2 super effective stabs on another, they could have had grass beat fire, and fire beat water etc if Chesnaughts dark would have beat Delphox psychic, who could beat Greninjas fighting etc.
So? It's not like they use both types at once, nor is any of the six types doubly effective on any of the three starters. The intent of starter mons is to create balance through Ro Sham Bo, and that balance is only upheld if there ISN'T a reverse triangle.
I mean, seriously, do you really want your rival battles to be EASIER? Jesus.