the rockets are not on the global cd and do more damage than an activated +spellpower trinket + EM instantlavaburst.
you can use the rockets with your EM instantlavaburst at the same time and they can crit for ~2,4k (depends on resilence)
and you can blow up a rogue with low health even if he had cloak of shadows activated, the rockets go instant through this fucking cloak in the face of this damn rogue D (with other words: he cant resist the pyro rockets with cloak of shadows)
i payed ~ 7k gold for engineering 1-425 on my 4 toons but you only need 400 for the pyrorockets. i love them couse they are not on the global cd and have only 45sec cooldown so you can kill another low life target for example with frostshock+pyrorockets
and the best: you can cast the pyrorockets while you are in ghostwolf and the pyrorockets have 45yards range