For Imp Swarm mainly if u uses it - only for shorty fights like Butcher and Tectus, if u not, uses for Demonbolt shortened CD and generates some starter fury.
I use it for Demonbolt dump. It's nice, because when you use the glyph of Dark Soul, 10 seconds sometimes is not enough for 4 demonbolts especially if you suddenly need to move during the dump. Haste from the trinket basically makes sure that all Demonbolts i throw are buffed with DS.