Shouldn't the hook_mail implementation check $key before alter?

From the API page: "Note that hook_mail() is not actually a hook. It is only called for a single module, the module named in the first argument of drupal_mail(). So it's a callback of a type, but not a hook."

So in reality no the key doesn't need to be checked. The key would be used when a module defines and sends multiple emails in which case using the key would allow for logic in hook_mail to be specific to the email being sent.