With JSON you use a programming language to transform your data. You also use a programming language to validate your data. Validating and transforming data are some of the most basic capabilities of all general-purpose programming languages, and reinventing these facilities badly in a not-exactly-programming-language like XSLT or XML Schema is a waste of time.
This is same as saying that one shouldn't use parser generators or even describe the grammar of a language in Backus-Naur form. Any language is usually very well equipped with tools to read strings and BNF is absolutely not a programming language. Is it a waste of time?
If you validate a couple of document formats, then maybe, but if you need to validate many different formats on a regular basis you'll soon want to split your program into 1) a language that describes the rules and 2) an interpreter that takes such a description and validates a document against it. DTD, XML Schema, and Relax NG are just such tools.
