I have a question on this subject as I've never tried adding a contact to BBM via email. What if the email address is linked to more than one device, would each device that has that email associated with it get an invite?
I've always added by pin, purly for the fact that some of my friends aren't as clued up on BlackBerrys as alot of the folks in here, so if they saw the email on their pc client (outlook/notes) then they wouldn't really be sure what it was for.
Email is less secure than PIN, but at the end of the day, it's not a big issue.
I'm not honestly sure what would happen, I suppose it would probably let you add them on the first BB, but then refuse on the second, but that's just a guess, might be worth trying one day!