Personally I only use ReimuB over the others because I suck too much to survive if I don't. Speaking Lunatic of course, where I suck too much to survive even WITH ReimuB. With weak shots the bosses will wreck me, MarisaB is cheating, and I can't get used to MarisaC. But on lower difficulties I like to go with ReimuA/C just because they're weaker, and because ReimuC seems really underused. Nevermind MarisaA, she's even more awkward than MarisaC.
But yeah, on my reasoning for stgaes, I've always been garbage at dodging the stage 4 post-Momiji wave (not that homing helps at all here <_<) and PoCing on the pre-Aya bats (helps here!!)... I also bomb a lot on stage 5, which I find very irritating (even if do I get most of that power back from point items). There seems to always be at least one stage of a game that likes to fuck with me, and homing/spread shots usually help me out with that a little. They're also nice when a new game comes out to get a feel for it, but that's another thing altogether.