By Bismarck.Ariyon 2013-06-25 15:16:51
Generally speaking, for most endgame events you'll probably be sitting in a single role- nuking, stunning, healing, etc- so constantly shifting between Light/Dark Arts isn't really an issue. (not that the recast timers would allow it)
If you find yourself in a situation where you need quick access to a single spell from the other side, there's always Enlightenment. (Say, if you're nuking/stunning and your RR wears off, or a melee bites it and needs a raise and the WHM is too busy curing to Arise)
I suppose a good rule of thumb would be if you're only going to be using a particular Art briefly (like, 10~ seconds), and don't require any specific stratagems from it, you probably don't need to switch over unless both timers are up and you feel like being thorough.
Edit: Possible exceptions for this could be swapping to Light Arts to Regen someone so you get maximum potency, or jumping to Dark Arts for a Drain/Aspir so you actually get something back from it.