Just to be clear, we consider the unbendable arm valuable for forward rolls and that's why we teach it at the start. I don't know much about judo ukemi, but my view of ukemi in aikido is the same as my view of aikido waza - it is never perfect and can always be improved upon. Maybe "lifetime" is an exaggeration, but it sure appears that way.
Hi Michael, as far as I know, Aikido ukemi wasn't developed by the founder but with minimal modifications imported by various teachers from Judo. Here is the first google hit on my search on Judo ukemi: https://www.youtube.com/watch?v=iKE7WS8N3Kw
Do you do something different?