Growing up, we used to get butter with jam on it, or more common, butter with sugared cinnamon on our bread or toast.
I've heard of the 'spread to prevent moisture leeching into the bread" thing.
I grew up with Kraft's Mayo, and could not stand the taste. Still don't, but when other brands finally became available in our area, I had to admit to my husband that I liked the other brands taste. But I still tend away from mayo, and am staunchly in favor of mustard, almost any kind of it, even the yellow stuff. We do have a wasabi mustard that is really good.