No, SMTP subscriber (or internet subscriber in older lingo) do not use licenses. They do not have mailboxes and can't call into the system, of course, so no license is required.
They are normally used to take messages for external folks and send them as emails with WAV attachments to an external SMTP address (i.e. for contractors you want internal folks to be able to leave messages for).
They also appear in the name lookup handlers if you want and can have transfer rules and such so you can use them to route calls to folks off site our outside the company or the like.
Lots of handy uses for SMTP users...
