"Double jack is the best option when there is no strong engine"
Double jack is strong, but there are almost always slight adjustments to be made. Sometime its buying 1 jack and then a terminal draw card or two, sometimes its buying a warehouse and a festival because they combo well.
"Jack of All trades is a boring card and never goes in engines"
Jack does a lot of things. Don't discount them. If there is no other way to trash estates, jack can do that for you, and replace them with silver. If building an engine, you can use trash for benefit later to get rid of the jack. If you have a way to trash copper, like spice merchant, it can be nice to build up your economy with the silvers...
Jack can even be an engine component itself. With disappearing villages or hamlet, it does draw up to 5. I played a game with fishing village and ambassador, and this was quite amazing and fast. Jack+hamlet can make a weird sort of money/engine hybrid that is definitely faster than double jack alone.
Engine building is tricky with jack around, because you have to be fast(<15 turns for the whole game often), and attacks are going to be less effective, and stalling is going to be less effective, because Jack doesn't suffer much from greening. Still, if you're clever, there's a lot of interesting things to do with Jack.