People regularly recommend other solutions over MathJax due to performance, and at least on complex pages there's a noticeable delay for it finish processing everything on the page. Is it worth it to do a native implementation because of that? maybe not, but there are issues with it.
Even the most optimized JavaScript code will be less performant than a fully declarative language that can be directly interpreted by the layout engine.
From a user standpoint, it's ridiculous that I need to have JavaScript enabled, so the browser can download and compile a separate runtime, that itself reparses the page, just so I can look at a static documents with some math symbols.
Lastly, I think a common standard for math representation is valuable for the same reasons "official" <section> and <article> elements are valuable: They offer a common data model that tools, extensions and search engines can work on to provide extra functionality. A "de-facto standard" like MathJax doesn't provide this, because there is no requirement that two different sites use the same representation. The only requirement is that they put up something which the particular version of MathJax they embedded can understand. This makes things a lot harder for tools.
