I do think the logic for Impa as from Diddy Kong is astounding.
Not only would she not add anything that Sheik can't already do, but Sheik was based off of her anyway. Impa isn't nearly as important as many people make her out to be.
In OoT, she plays essentially the same role as 6 others do as a sage. That cameo appearance in the painted glass in Wind Waker? All the sages were in there. It's nothing special.
The only thing she has going for her is Skyward Sword. You could say the same for Sheik in OoT, but at the same time, Sheik was included because of the perfect opportunity to create a phenomenal moveset for Zelda. Phenomenal in uniqueness, not necessarily execution.
Impa is honestly nothing special.
Also just noticed I have 2500 posts. Yippee
You know, sometimes when I break these things down, I feel like I should either make my own thread like Chrono's, or do some sort of video series. But i'm too friggin lazy. Blah.