I know someone who has their biometrics set up for quicker passage through the lines, and then first time out couldn't get their fingerprints to scan properly (because their hands were too dry, I think). The official in charge very calmly went through the rest of the paperwork and processed it just fine without the fingerprints, sending them through.
Fingerprints are fast and easy, this person would have been through quicker if the scanning had worked, but they cannot be required until they are completely hundred percent reliable - not just in scanning but in the machine not freezing up or throwing errors or anything.
Right now, fingerprints will either not-match based on factors that affect the skin - including hydration, blood flow, injury or illness (rashes or the like), swelling, or pruning, or even just dirty, exact way it is pressed or moved against the scanner, and others - or else it will sometimes match falsely based on such errors. Either false negatives, or false positives... or both, that is also a possibility. I had all the same issues when I had a fingerprint scanner on my computer - it was mostly reliable, but not always, and one had to be careful and have other ways in - and I don't imagine the tech has become hundred-percent reliable since then.
So there will be options for those cases where it isn't working - even if, over time, the scanners become reliable enough to make this rather rare, it will still have to be somehow possible to deal with outlying oddball cases.