I'll take the 100s or the 120s, thanks. You can dive your AL80s, no problem. I would have no with problem with them. In fact, for 4 dives a day, might go for it.
I don't need 120's to make 90 minute dives but some divers do. That's the beauty of having choices. Scuba cylinders are not one size fits all. When divers need more air, we have the tanks available to them. We offer 120 as a rental through Aico and we own 100, 80, 60, and even 40's.