Well I happen to know the answer, but I'll leave others to guess first:
You need a Jack for 2 reasons: to deal with estates/curses, and to get economy up to what you need for KC, which allows you to use your $5's to buy laboratories instead of building up your economy.
and it brings up an interesting strategic point:
One Jack does not substantially dilute one's deck. Often people are afraid to use jack in an engine. However, if you are trashing an estate or a curse when you play it, it is not diluting your deck at all, rather it is just replacing a bad card with a better card.