I've gotten bit by trailing commas enough times (both manual edits and writing generators) that I absolutely expect any reasonable syntax to tolerate them. It's just so much easier and more consistent to tolerate them.
That's what Clojure got right. Comma is whitespace. When you print a datastructure, it has commas. But they don't effect reading of that structure. Its brilliant and practical.
As importantly, it reduces the noise in diffs when an item is added to a sequence.
You can use prefix separators for that. Haskell-style.
Doesn't that just push the problem to the other side of the list?
It does. However, it’s much more common to edit the end of a list, in my experience. Still, a syntax that is entirely uniform (like trailing commas) is preferable, in my opinion.