> You fixed something from the immediately-buildable tarball that you'd rebuilt and tested
That entirely depends on whether the necessary libraries and headers are installed on the system you're using.
> even something that didn't need a rebuild (config, doc, interpreted source), and sent the change, perhaps using M-x diff-backup
Unless someone uses emacs and gnus or something similar, email clients could end up mangling the diff unless it was sent as an attachment (which would make reviewing the code and posting an inline response more difficult).
I was fine with that as a maintainer too, on three very active major projects at one time (using a 48Mb Pentium 1). A maintainer's job is partly to make it easy for contributors, not for the maintainer, but tending the contributors will help in the end.