Privacy and security is what mobile company focus on now a day and they got a pretty interest features for unlock their devices. First the finger print scanner which is made famous by the iPhone 5s and every major smartphone adopt that technology but Microsoft approach the different way by announcing the Windows hello feature that authenticate your identity with your eye balls as we seen the feature work on the Surface Pro 4, Surface Book, Lumia 950 and Lumia 950XL. The iris scanner use the infrared light to scan your eyes print and it work in any lighting condition
To answer that question I have to say sure it's nice to have that feature but it is a must? no. Because the technology is not as mature as fingerprint scanner and some time it's clumsy to hold your phone up to your eyes to scan it. With fingerprint reader you can unlock your phone at the time you pick it up or take it out of your pocket (eg. Nexus 6P) without having to look at the scanner. But when your thumb is wet or you're wearing gloves in a cold day the Iris scanner will come very handy in those situation.
For the conclusion, I think the iris scanner is optional as it is now and having to choose with fingerprint sensor I would choose fingerprint sensor over it but thing might change it the future when the iris scanner is more mature and I'm waiting to see how great it is.
Windows Hello on Lumia
And now the Galaxy Note 7 also adopt this technology for the first time in their Galaxy line. So the question is that is it worth it to have an iris scanner for your phone?
Galaxy Note 7 Scanning Iris
For the conclusion, I think the iris scanner is optional as it is now and having to choose with fingerprint sensor I would choose fingerprint sensor over it but thing might change it the future when the iris scanner is more mature and I'm waiting to see how great it is.