We are talking about cell phones here not a bank vault.I disagree with you about Iris Scanners (and retina), which I'm familiar, Security analysts considers all biometric recognition as a 2nd verification protocol to reinforce main security protocol (joint with passords/and a hardware key), this is for SERIOUS SECURITY, but Apple (as Samsung seems will do) offered it as gimmic, just to help lazy users to keep "locked" their devices.
If Apple does offer, an option to require a short password (4 digit) and figerprint recognition this is an safer measure, but FP alone NOT.
Iris Scan isnt that complex, actually very similar to fp scan, and is done on IR spectrum, the advantage is that you cant clone an iris. Similar to Retina scan (wich requires a more complex but similar sensor), btw no biometric sensor is capable to read your WILL, so are only a 2nd verification protocol.
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So they are just a security gimmic.
Text passwords have always been a poor form of security. A good password is hard to remember, so most users pick a bad password making it's effectiveness as a security measure questionable. If you've ever managed a user database you'd know just how often people will use "1234" or "1111" as a password. It's been mathematically proven that enforcing recommended passwords like "1X$%34@#" are actually less secure (as in quicker to brute force) then something like "ilikeboats".
The problem was users not using a pass code or using a dead simple pass code for convenience. Apple noticed a large percentage of it's customers fell into one of those two categories and added FP to make it more convenient to secure your phone. The FP scanner has been successful in doing that. So while you may call it a gimmick for a lot of users their security (or previously lack there of) has been improved.