Having ridden these bikes and having no problem looking over my sholder. I can't see any reason to install a mirror, it's only something you will miss if you have one on your bike, and lets be honest how many people do. (i don't have an issue with mirrors, just that from a general obvservation, the majority of cyclists don't have mirros)
You assume there that the hire bikes will be used by people who have their own bikes, or at least use them regularly. If you want to get people out of cars for short trips, then make the bikes like the cars - with mirrors for easy rear views....
I have mirrors on the trike - it's much more difficult to look right round from the seat. Everytime I ride the trike and then get back on the upright, I have to readjust to not having the mirror to just glance into as I go along. On the trike, they are invaluable, and I like being able to monitor the road behind constantly without turning my head - I still do lifesavers of course. On the upright, I can glance round more easily, so I do that instead.
Mirrors can also be very useful for people like my Mum, with poor peripheral vision, or loss of sight in one eye - to see clearly behind her, she'd have to be looking round at virtually 180 degrees.
I tried a Bike Eye, and it was useless - but then they do say it's not meant for a bike with rack and panniers, as they block, or complicate the view to the point that you can't take it in quickly.
Being able to look round for a long time without wobbling is a skill, and one useful perhaps on a long straight road. In town, I fear you might be looking back so carefully, you'd run into the idiot stepping or pulling out in front of you....