In case anyone sees this: I initially agreed with you, thinking that pure UTF-8 validation is a bit more of a niche than might be expected. But if I'm not mistaken, I think at least for JSON validation, the UTF-8 validation can happen completely independently of the parsing. I don't think any control characters (braces, commas, quotes, escapes, etc) will change the validity of the UTF, or vice versa. So it might be beneficial (both for speed and security, as the sibling comment notes) to validate chunks of input as UTF-8, then pass them off to the parser, which now doesn't need to deal with validation.

Maybe this could be applied to XML, but that's quite a behemoth of a standard--I really wouldn't be surprised if there was a way to switch encodings mid-stream. I have no idea though...