if we were outputting HTML4, it might cause
∗genuine∗ problems - e.g. if you need XHTML in order to mix in MathML or SVG
.. to do which you must serve as application/xhtml+xml.
That said, I agree with his points. And personally I wish the browsers would relax the restrictions on when they will agree to render SVG. I see no good reason why they refuse to do svg in text/html mode, considering the exceptions they make for seemingly everything else.
This probably breaks major assumptions inside the browser engines. Rendering most image types is a one-and-done type deal, just setting up a few calls to libjpeg or libpng, but SVG actually has to be XML-parsed and rendered element by element on page load. To do it in text/html, you'd need to hook together the html parser with the XML parser with the image generator. That's a lot of work to break the rules!