There are over 8 different keys from what I can see for all the different emails, and that's just for Base - registration email, reset password, resent verification email etc. Then you would also have to add the information to any plugin that also sent emails. It doesn't make sense to have to modify each language key everytime you need to change something.
The way I see it would be a text area in the admin area, maybe under content settings, or a new section called email for smtp and signature settings.
This text/html in this signature box would then automatically be appended to every outgoing email.
This would mean that you can easily make any changes once and not have to worry about missing a language key that you manually have to find yourself.
I can then easily add my company name, company number and any other details such as a support email address to every message without worry. I'm positive other users would find this helpful too.