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Disc would also be more valuable in early lvl instances than a shadow build.
While that may be true, as a caster you could easily go into any instance up to about SMGY with no talents and completely naked and still do fine.
Disc is the survival spec - you won't die easily at all. Shadow is the damage spec, once you get Flay and especially once you start getting Weaving, Darkness, and SF, things speed way up. Holy is the instance-grinding spec, or for the person who doesn't mind spending as much time drinking as they do killing mobs.