Consider the attached setup, say to craft gold ingots to blocks. The lectern gives redstone level 8. As the crafter reaches 9 slots, the observer is moved over the crafter and powers the block on top of it. The crafter spits a gold block into the dropper, but the dropper, which is quasi powered, never gets a block update.

Also it's rather curious that the crafter can't be quasi powered, I believe this makes some contraptions harder than they need to be.