MarisaB is awesome.
1) Focus
2) Stay under the boss
3) Basically don't move
...and you can drain away enemy lifebars very quickly. She's like SakuyA, except preferring cheap damage over cheap range. (Their Cherry rates are comparable IIRC.)
Which MarisaA can also do, only with more power, (slightly) more range, a higher Cherry rate, and a lower Cherry loss for bombing/dying. Oh, and MarisaA can actually do damage without focusing :V
SakuyaA's Cherry rate is abysmal, having a comparable rate to hers is
not something to be proud of, especially considering how both shots need to be focused to do anything resembling damage. Focusing cuts their Cherry gain to roughly 10% IIRC, and although that applies to every shot in the game, it's especially bad when their rate is so terrible already.
ReimuA is good for "getting into the game"? She's really not; that's what SakuyA is for :V
And if that was your best reason for justifying her existence, yes, she's the worst in the game.
ReimuA and SakuyaA are terrible shottypes overall, but they're both very good for beginners just learning the basics of Touhou. Auto-homing shots allow the player to focus almost purely on dodging, and not have to worry about hitting the enemies until they've improved their dodging skills. Outside of that, they're both garbage, I don't think anyone's going to debate that.
I guess my problem with MarisaB is not that she's a bad shottype, she just doesn't have anything super-useful that only she can do.
-ReimuB is solidly powerful, has good range, a small hitbox, and decent Cherry
-MarisaA does the most damage in the game per shot and has the highest Cherry gain
-SakuyaB is broken in every way and fantastic for both scoring and survival
-ReimuA and SakuyaA are good for learning the game, at which point it's better to move onto a better shottype
I just can't find anything MarisaB does that another shot can't do much better. She's strong when she's focused and has powerful bombs, that's not up for debate, but that's about it.