Mixing Runes

[Einalem]

because why would anyone choose 90 wind resistance over 90 wind AND 90 dark resistance?

Offensive Reasons.
Lunge/Gambit/Rayke all reward Runic Linearity.
You want 3 of the same type of Runes here.

The issues cited in general with Rune Fencer are Defensive issues, not Offensive ones.
It currently takes 10~15 seconds to swap out 3 rune types over 3 /ja activations.
Of course 90 Wind and Dark is better, but the point of the post you're quoting is to show that there are methods that could be employed to give RUN more Defensive flexibility.
In fact, there's an entirely second method right after that first line you quoted that's deleted out.