You should always have points in gag order for the increased shield slam damage. Imp spell reflect is too situational imo. A lot of times the spells either won't kill you, or the points are better spent elsewhere. Most fights if damage is to hit ranged, it's AoE and imp spell reflect cannot help them, though I can see it useful in heroics (and PvP).
There may be some fights where the extra 4% chance to not be hit ends up being good, such as Sindragosa, but those fights also might require more cooldowns, and therefore imp disciplines may be better.
So in recap, imp spell reflect is good situationally, but is a waste of talent points in most situations.
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