kaleo is really an extension on top of DUA signup codes that is just providing all the user level functionality so I think it is completely useful and not at all duplication to have an admin level invite thing in DUA.
As far as sending the email, I am ok with either. kaleo sends the email because it is creating the signup_code directly and calling send(), though I guess technically means DUA is sending it and kaleo is just calling a function in DUA that sends it.
The hooksets that Brian added recently gives the site developer really full control over email if it needs to be customized at the site level (like routing through something like Mandrill or sending HTML emails, etc.).
So I guess I am leaning towards keeping the emails sent from DUA and replicating the hookset functionality in other apps that send emails.