So basically, Kasen deserves a theme and game appearance because you like WaHH, whereas the three fairies didn't because you don't like SaBND. I like all the print media (even CoLA), but I wouldn't say either WaHH or SaBND are particularly "illuminating" to the characters. Both give fun characterizations of characters by showing how their canon personalities act, but neither really develops anyone beyond their main characters. I'm not saying I'd hate seeing Kasen in a game with her own theme, but saying Kasen deserves it because you like how she's handled better than others seems somewhat hypocritical.
Kasen has an interesting background as a hermit, an intriguing character design, and a relatively new series with only a handful of installments so far. SaBND had more issues than WaHH and did absolutely nothing interesting with them, and WaHH has the potential to do a lot more, even though it hasn't gone that far yet. Neither is particularly illuminating yet, but Kasen has a certain enigmatic air regarding her appearance (namely, her right arm) that the Three Worthless Fairies missed out on.
Am I biased? Well,
duh. I don't quite see how that makes me hypocritical, though; doesn't it make perfect sense to want to see your favoured characters more than others?
Why you gotta be hating Ruro?
Look, you can have your slice of life with Three Moronic Fairies whenever you want. I, for one, would like to see more of Kasen punching out criminal spirits with her delicious bandaged right arm. Personal tastes and all.
I honestly remember very little of the details of SaBND. This is probably because nothing interesting ever happened. I do remember really liking Makoto Hirasaka's take on the visuals of Gensokyo and slightly snickering at how the canon design of the SDM's architecture changed from SaBND to SSiB, but apart from that, it was still really dull. Slice-of-life is not what keeps me interested. CoLA is even worse in that regard, and I will never, ever understand why people bother to read it for any reason than it being canon that they should know. I would say I was sorry for not being able to stand boring written works, but that would be lying.
For the record, of all ZUN's written works, it's his stories about Maribel and Renko that interest me most-- they strike just the right balance of adventure and enigma and fun for me. Second place is Inaba
2, which was just hilarious.