> I don't know about the security vulnerabilities,
About 14 months ago, it caused some of the most serious vulnerabilities in the Ruby on Rails world ever: http://tenderlovemaking.com/2013/02/06/yaml-f7u12.html
> why not just use JSON?
JSON is not really human-editable. Those quotes and commas, ugh! Also, JSON lacks comments.
The vulnerabilities in YAML (which is a superset of JSON, by the way) point at why YAML and JSON both aren't appropriate for configuration: they are _serialization_ formats. Configuration isn't what they're built for.
And you're right, it's really just not a huge deal in any way. Especially once we have `cargo project` to autogenerate the basics.