Yeah, I have to agree with Sol.
Perhaps ironically, the very same flaws that Akito points out is precisely why Achiga-hen is a show that any true Saki anime fan should watch.
It's true that Achiga-hen doesn't stand on its own all that well... and that's because I don't think it's meant to. It's more supplementary material than true spinoff, imo. But its supplementary material is strong, compelling, and informative enough that I also would consider it required viewing for fans of the Saki franchise.
My impression is that the three most important characters in Saki-verse are Nodoka and the Miyanaga sisters. Achiga-hen goes into considerable detail over Nodoka's personal backstory, and really highlights how Nodoka is the Kevin Bacon of the high school mahjong world.
Teru is showcased brilliantly in Achiga-hen, and so getting to see her here will go a long way to fully appreciating Teru in any future Saki anime shows, imo.
And there are some nice subtle hints about the strained sibling relationship between Teru and Saki and what might be fueling it.
Another thing I want to add, Akito, is that I actually think you were a bit too hard on the Achiga-hen protagonists. I myself initially had difficulties telling Kuro and Ako apart, but that had at least as much to with visual similarities as personality ones. Thankfully, Kuro was in the Teru match, and that alone will forever distinguish her from Ako for me.
I also think that personality-wise Shizuno is pretty good and distinctive. She's a fiercely fiery competitor with real genki girl spunk. And she wears virtually nothing from the waist down... how can she
not stand out to you?
All of that being said, you raise some valid points against Achiga-hen. I do think that Achiga-hen is the classic case of an anime adhering
too tightly to its source material, causing pacing problems for the anime. It might have been more effective if they had wrote some anime original material to stretch out some of the mahjong matches featured in the first few episodes.