Your point about immutable keys is important, I think. A "natural key" can mean different things, as I understand the term. You could call a person's social security number a natural key, which for the record is a bad idea as they can change . But you could also call the combination of user_id and reservation_id a composite natural key, which is immutable and not so bad in my book. I'd still use a surrogate key though.
It might seem like I care about this more than I actually do. I've just found over the years of writing systems that I fairly often regretted not having a surrogate key on a table, but I've never once regretted adding a surrogate key that ended up not being essential.
What I've seen happen is that the data model expands/changes and I need to reference a join table. I might just start out referencing it by the composite natural key. Then the model changes again, and now I need to add another piece of data that changes the natural key. For example, maybe the natural key was (user_id, reservation_id), but now it's (user_id, reservation_id, membership_type_id). Now I need to think about anywhere I referenced that join table by the natural key.
