I have also seen non-spam games that are submitted to multiple jams, which the reason for should be obvious: Creators are looking for ways to gain exposure, and for good reason... they worked hard on their game and if it isn't breaking any rules, it is in their best interest to get more eyes on it.
That said, it dilutes the uniqueness of each jam. To me, a game jam should be an isolated event, not a database of everything that fits the prompt. It should be like going to a specific location and crafting something on-site. You wouldn't bring that creation to the next craft-jam and say "look, I made this here, too!" This just turns into a snowball of duplicate entries... as an audience member, I would rather see fewer submissions, with each one being unique to the single event that it was made for.