Ah, the sounds of someone who has never done any work with regular, technically illiterate users and has probably never dealt with a high-volume, multiply-forwarded email environment.
Two main reasons to include your email address in your email signature: many users have no clue how to find your email address in the email client. I’d say 30-40% don’t, from having supported users in the past.
And depending on how the sender’s email server and client is set up, and how your email server and client is set up, forwarded email is sometimes stripped of the email address so you can only see the person’s name. If the original sender or senders’ info is not in your address book or GAL (etc.), then you have no idea of anyone’s address beyond the closest-proximity email.
So yes, in a business environment it is always smart to include your email address in your signature. It helps technically-illiterate users and it is a dependable way to ensure that even email forwarded to others always contains your email address.