Honestly I get that there an advantage in respect to tooling and this might ease integration into existing system, but I can't help feel that this is introducing an extra level of complexity that you would only find acceptable if you at IBM customer level scale.

The ability to introduce at least some type safety seems nice though.

It may be somewhat problematic that it doesn't actually support all JSON, because the characters that are permitted (even with escapes) in XML 1.0 text do not include all characters that can appear in JSON values, so if you use it on legal JSON that isn't constrained to be XML 1.0 compatible, it will do something wrong (the docs aren't clear on whether it will fail or just drop the offending characters), so the only place that it seems safe to use is in a constrained internal environment where you control "JSON" to mean "JSON using XML-1.0-safe characters".