This issue went from created to committed in less than an hour. Changes to the FileSystem API should require more notice and explanation than was provided in this JIRA, though this looks like a reasonable convenience method to me.
Hmm.. makes sense to wait longer for such changes. The main reason for having this API is to take care of libhdfs. There is an API in libhdfs that tries to connect on behalf of users. In order to support that API, I added the method in FileSystem. The other option would be to create an UGI object and pass it to newInstance but that would complicate things a bit (to do with creating UGI objects in C code, etc.).