imp demo shout is useful in conjunction with curse of recklessness.
bosses have 350ap; for mobs, 2ap = 1dps
2/5 imp demo is a -348ap debuff, thus reducing mobs' ap to near-zero.
curse of recklessness is a slight ap buff to the mob, plus an -800 armor debuff (which does stack with sunder.)
5/5 imp demo is enough to reduce a mob with curse of recklessness to 0ap.
gruul is pretty much the first fight where 5/5 imp demo + curse of recklessness can come in really handy, especially in the first couple kills.