Id have responded quickly had it been posted there, it is my boomark.
As for /SCH, jumping arts is useful, but more often then not you can stay in light arts pretty much indefinitely. Dark Arts doesn't really give us that large of a boost except for some AoE CC, Graviga primarily (if you need sleepga you should just /BLM). Everything else from dark arts is kind of meh, the element ACC boost is nice but with how much MACC you get via atma's it is mostly redundant. Light arts is good, because it gives you -na's as well as some longer MP duration. As well as AoE Stoneskin, and IMO the best AoE of Enspells, (an additional 25 DMG with a solid enhancing set on a group of melee is a nice bonus.)
As for the inevitable post in regards to casting timers and mp cost increase of cross art casting. RDM can pertty much cap recast alone. With 50% in fast cast (25% recast) 15% haste buff, and 10 more haste required in gear (easily done). You will notice a little bit longer times by comparison to not having it, but in reality it is not as harsh on a RDM as say a SCH being caught in the wrong art. The MP increase can build up over time, but really with high teens in refresh, and convert, as well as the MP saved from casting cures, hastes, refreshes in light arts it pretty much balances itself out, and again does not hit RDM very hard.
You can full time light arts, and swap as needed to Dark arts. Dark arts is mostly redundant, unless you know 100% for sure you will be nuking or CCing 100% of the time. Otherwise you may as well just nuke and CC in light arts, with minimal drawback.
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