I've enabled Debug mode in lightning components.
When I tried to use
$A.log("This is a log message");
You shouldn't have to turn on anything else as far as I can tell.
aura_proddebug.jsis loaded and used and when it is not checked
aura_prod.jsis loaded and used and the debug version isn't even loaded. (That's consistent with the documentation and expected).
You can step through the
$A.$ns$.$Aura$.prototype.$logInternal$is called, but doesn't do anything.
If you need to log for now, you could always write your own log function that delegates to console.log, if present, and then later change the function implementation to use $A.log.
$A.warning doesn't work also, which makes sense because it is essentially the same function as the log/info level logging, but just at a different level. $A.error works, but it is completely different in that it pops the exception overlay.