It has to be useful for colony games, I mean it doesn't really hurt your speed dramatically in province games. In addition, I don't really want to buy double-jack in colony games -- the silvers just aren't that good. 1 jack and one spice merchant though, will help me play my jack more early, get rid of copper, and later on can always get rid of silver. This will make golds/plats come up a lot more, which has to be good.
As for engines, spice and jack definitely combo well, especially when there's no other way of killing estates.