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About the mage, yes they still spec in imp scortch but they can swap to an other glyph and not worry about scorch in there rotation increasing there dps slightly.
Elemental shaman can use other totem like fire elem and such without worrying about raid loosing the +SP dmg
the crit part of the totem is covered like other said by the pally. it's rare you have multiple boss in a fight.
about the melee unfriendly fight, with Fel synergy and natural pet 75% avoidance, i can leave my pet in 99% of the time on every boss fight and not worry about him. I rarely have to resummon him.
I would say Demo in 10 man is not realy worth it. In 25 man if you have someone to cover the 3% crit, even if you have an elemental shaman if your lock has enoght SP to overcome the shaman +sp 1 Demo lock in the raid is more beneficial since your mage also gain from not having to keep scorch and can glyph out of scorch.
the exact dps gain/lost can't realy be calculated, it's more of a, will it benefit the raid enoght or not question.