/sch sacrifices too much damage vs /rdm to be viable imo. The limited utility from light arts doesn't make up the difference, unless you're flat out hurting for mages, in which case you may as well go /whm for curaga2 and tranquil heart.
Remember, Firestorm only gives single day/weather bonus, which is 10% or 1.1 to your QD calculation, and all obi does is guarantee proc. Firestorm + Obi is a 1.1 multiplier to your damage, mab2 trait from /rdm is 1.24, 1.27 if you count the belt slot you use for obi on qd. That's like giving up hecate's+novio+nimue's fulltime, and tossing in extra cast time every 3 min.
The utility from light arts is pretty extreme if you are in a DD party. But, that aside.
What utility beyond aoe /na spells, which your Whm or Sch (VW groups should have both for cures/triggers, and there's no reason one or the other shouldn't be main healing the DD party) should already be doing if you're in a DD party? If you have to come a sub to assist that, the best utility for a /mage Cor in VW is clutch cures after devastating aoes, and /whm is better for that. If for some odd reason you are the primary source of Cures/buffs/na spells in your party, you get Cure IV, Curaga1/2, Cura, Shellra2/Protectra3, Stona, Divine Seal, and bar spells all outweigh the benefits of Light Arts. That's before taking into account the limits imposed by Strategem charges, ja delay slowing down your aoe cures, and Light Arts bonuses not stacking with Strategem bonuses.
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Magic attack bonus from /rdm is equal to +24. Depending on your current build for wildfire, a straight 10% boost to damage, after applying your current MAB from wiz roll/gear, can often exceed /rdm, and will always come close. Imo, the utility from /sch will far outweigh the fractional and only occasional increase from /rdm.
First off, I was talking about QD, hence the QD mentioned in my post, but I'll bite for Wildfire as well. You're under the mistaken assumption of how Wildfire is calculated, which is part of why you're wrong. Affinity from staff, affinity from weather bonus, and mab are all added as multipliers after the initial base damage calculation.
For Wildfire:
[floor(5.5 * (Lv + 2 + floor(@ * floor(AGI * .6))))] + [2 * (Your AGI-Enemy INT)] * (1 + (MAB / 100)) * (Elemental Staff bonus) * (Day/Weather Bonus)
For QD:
(2 * (Gun base DMG + bullet base DMG + Corsair's Tricorne bonus)) * (1 + (MAB / 100)) * (Elemental Staff bonus) * (Day/Weather Bonus)
Since MAB is calculated as (1+(MAB/100), the MAB from Rdm is 1.24. Assuming 23 gear mab + ascetic's (25 mab)+ /rdm is the maximum reasonable estimate in VW, that's (1+(72/100))= 1.72 as the max reasonable multiplier, 1.88 if you chose to use Wizard's roll and hit 11.
For day/weather bonus, you get 10% for matching affinity using firestorm+obi. For calculating magic damage in FFXI, that would be 1.0+ 0.1 for magic matching single weather, which gives us 1.1 as the day/weather multiplier added for Obi+Firestorm.
Using sets that lean towards mab over stp just for argument's sake, since you brought up decreasing returns. Increasing amounts of MAB using Ascetic's and perfect Wizard's Roll (it's silly to use it for VW, but included to give an advantage to /sch). Leaving out staff because it's a separate multiplier from both day affinity and mab, so it's not pertinent to the difference between the two. I've also left in the mab that would lost on the belt slot for Obi on QD, and the agi that would have been lost on waist for WF to give /sch a further advantage.
Simplified QD Calculation with assumed dmg pre mab/affinity multipliers:
/sch 200*1.34*1.1 = 294.8
with ascetic's
/sch 200*1.59*1.1 = 349.8
with ascetic's and Wizard's
/sch 200*1.75*1.1 = 385
/rdm 200*1.58 = 316
with ascetic's
/rdm 200*1.83 = 366
with ascetic's and Wizard's
/rdm 200*1.99 = 398
Min increase in damage for /rdm over /sch = 3.33%
Max increase in damage for /rdm over /sch = 7.12%
Average increase in damage for /rdm over /sch = 5%
Simplified Wildfire Calculation with assumed dmg pre mab/affinity multipliers:
/sch 1000*1.23*1.1 = 1353
with ascetic's
/sch 1000*1.48*1.1 = 1628
with ascetic's and Wizard's
/sch 1000*1.64*1.1 = 1804
/rdm 1000*1.47 = 1470
with ascetic's
/rdm 1000*1.72 = 1720
with ascetic's and Wizard's
/rdm 1000*1.88 = 1880
Min increase in damage for /rdm over /sch = 4.21%
Max increase in damage for /rdm over /sch = 8.64%
Average damage increase for /rdm over /sch = 6%
Drop full time 11 Wizard's which no one uses and account for downtime on fanatic's and you're closer to 7% average WF increase.
Let's assume after your first ws it takes you 1 /ra or 1 qd to ws again, and you alternate from one to the other every cycle. That's 120(guestimate for the shot) + 1628= 1748 or 349 + 1628 = 1977 in a damage cycle, for an average 1802 per cycle damage /sch. 120 + 1720 = 1840 or 366 + 1720 = 2086 in a damage cycle, for an average 1963 per cycle damage for /rdm. That's a 9% increase in total damage for /rdm over /sch during your cycle.
/sch never exceeds /rdm in damage which is the only thing I compared them on, and I don't consider what was shown above
in the worst possible case for /rdm to be a fractional or occasional when considered against the majority of incremental swaps we make in this game.
Again, if you want damage, go /sam or /rdm. If you want back line utility go /whm. It's easy to look at your alliance before you start and choose the best option, in almost all cases going for the middle ground with /sch isn't it. Having second rate healing abilities and third rate dd ability just to be able to do both is silly.