It's more "if your primary tool is a text editor, there is a complexity cost to context shifting, and for some simple tasks that cost isn't justified if you have a suitable method of representing what you want in text."

Just as with programming, polyglot programming has value, but switching languages within a project had a complexity cost that isn't always justified even when for dinner task taken alone a different language is marginally superior.

Graphics files are not both self-describing and inline.

For some applications, having something be both can be helpful.

Way off topic but this is one reason I was excited about Atom the text editor. Given its running inside a browser you should, theoretically, be able to show svg inline, even allow visual editing inline.