Well, the bonuses obtained from an S rank in any other Magic type are greater than the Latona Staff, which is about all you receive for getting an S Staff rank.
Pretty pointless, especially because Latona is just a Fortify with an added ability that is pretty useless.
But really, the likes of Moulder and Natasha are never going to enter combat anyway, let alone reach S Rank in Anima/Light. Latona, which is the only ranged staff that cures status effects and the only infinite range staff (important when you consider that most staff users are unlikely to have good magic), is pretty good in my eyes.
I love Seth. And Titania. And Marcus. All those great already promoted EXP drains they force to take up a lot on your team.
I didn't know you got a 5% crit/hit bonus for an S rank.
That's pretty awesome.
With that said, it may be preferable to have one character (Choose; Moulder, Natasha, or L'Arachel) to S-Rank staves, while everyone else S-Ranks Light/Anima. As has been said, you only get three uses ( or up to 9 with a Hammerne staff, or up to 15 with both Hammernes... which is just wasteful considering the weapons you could be repairing) out of the S-Rank Latona staff. So if you wanna keep your Hammernes for more important, awesome weapons for LA (Reginleif, Sacred Twins, etc), you'll end up with only three real uses of S-Rank staves. Which, you might only use it once for the final chapter, maybe? *shrug*
S-Rank Light isn't a whole lot better. The Tome, Ivaldi, is pretty much just for a Valkyrie (L'Arachel) or Sage, and a Sage would probably rather master Anima for the sake of Excalibur. Ivaldi has x2 damage against monsters, and that effect overrides a Bishop's ability to do x3 damage against monsters--meaning, it's useless for Bishops, who can do more damage with an A-Rank tome. Still, the 5% bonus crit/hit could well come in handy, who knows? Staves, of course, have no Crit or Hit percent(Well, unless you're using Status staves... but those are hellishly irritating to get).
Overall, if you choose to use Bishops, then Light mastery is generally better than Staves mastery, but you'll still want Staves mastery to be on either a Bishop or a Sage, so that Troubadour(And/or Mage Knight?) can master Light and use Ivaldi for extra damage. Over that still, though, it'd probably be better to just forget Ivaldi altogether and stick with Excalibur usage.
Also, this: Bishop+Purge=Epicsauce in the ruins. That is all.