I have not yet achieved the level of skill with peanut butter required to get all of it off the knife. Most, yes, but there's still a plainly visible amount left.
But then I'm gonna get peanut butter on the bread I'm going to put jelly on. What if the spoon gets contaminated, and then I put it back in the jar for the next person?
...Wait nevermind, that's not a problem, I use a squeeze bottle.