I agree, this seems like common sense, and I too am surprised the functionality is not yet there! Andreas made the Auto PM new users hack, I'm sure it wouldn't be too hard for someone who knew what they were doing (unlike me) to modify that to simply execute when the usergroup is updated to a new primary/secondary usergroup of subscribers.
Another Wish-List feature of mine is to require payment at registration! I simply am appalled that this hasn't been included yet for sites that are paid members only. I mean do we really expect people to register, verify email address and then visit the site scouring long enough to find the payments.php and purchase a subscription from an obscure link in the User CP?
Now you can make the link more obvious, put it in the welcome email/headers hack and such but I think that there should be an option to include that process in registration. Enter username, email and password along with options, choose paid subscription you want, enter credit card information or paypal information, process sale, then register the user with the system if this all works properly. One fluid multi-step system.