Don't get me wrong, if you have a good jam or preserves it's totally fine on a sandwich, and I wouldn't turn my nose up at it. However, peanut butter doesn't really need the help, and the extra sweetness from jelly is totally unnecessary. Toasted bread and peanut butter is all you need for a quality simple sandwich.
When I grew up, bread with butter already passed as a simple sandwich. Pbj is disgusting to me, while bread, butter and jam (or honey) is good as an open faced sandwich.
The flavor of butter can be the star of a sandwich, a baguette jambon beurre in france is fantastic.