As I said on IRC:
* we don’t care if we concatenate strings when needed (= when a message must be logged because of the gDEBUG setting) — in production mode, this will only happen to report missing l10n entities, which should be considered as a serious bug;
* if we don’t concatenate the arguments, the output of `consoleWarn('this ', 'is ', 'a ', 'test')` in Firefox will be `"this " "is " "a " "test"` instead of `"this is a test"`.

Let’s not micro-optimize for cases that should never happen in production, and let’s keep readable error messages when needed.