Right now Beeper Mini only works without registering your phone number, and I'd be ok if that's all I ever got. In fact, I hope they make number registration optional if they do get that working again.
I still have my Macbook if/when Beeper is cut off again.
It definitely might be better at the moment without registering your number as then you won't have messages disappear into nothingness if the service goes down again.
I missed a few messages when I switched from iPhone to Android because I hadn't deregistered my number from Apple.
I don't think I'd use my existing Apple ID for this -- probably easy enough to create a new one with a new email.
I still have my Macbook if/when Beeper is cut off again.