Just to make it explicit (and without inserting any personal judgement into the conversation myself): JSON parsers should reject things like trailing commas after final array elements because it will encourage people to emit trailing commas?
Having asked the question (and now explicitly freeing myself to talk values) it's new to me -- a solid and rare objection to the Robustness Principle. Maybe common enough in these sorts of discussions, though? Anyway, partial as I might be to trailing commas, I do quite like the "JSON's universality shall not be compromised" argument.
Accepting trailing commas in JSON isn't as big a deal as having two different opinions about what a valid document is. But you might think a trailing comma could indicate a hand-edited document that's missing an important field or array element.
If you put out JSON that gets misparsed, you either generated invalid JSON, or the parser is faulty. Nothing around that.
This has nothing to do with whether parsers have flexibility to accept additional constructs, which is extremely common for a parser to do.
Accepting invalid or ambiguous or undefined JSON is not an acceptable behavior. It means bugs get swallowed up and you can't reliably round trip data.