Biometric data cannot be kept secret, nor can it be changed if compromised. It is only useful in situations where everyone who has, or can claim to have, scanning apparatus is trustworthy. For something like the entrance to Fort Knox, this is a reasonable assumption. For something like making credit purchases, it is not.
Biometric data relies on the person being present, do it couldn't be applied to routine credit purchases, which are often done via Internet or phone. Convictions based on biometric data would obviously be subject to verification of identity as part of the court proceedings, so nobody's going to end up doing prison time or getting deported as a result of some airline having a faulty scanner. And if once in a while a legitimate voter got denied the opportunity to vote, that would hardly be worse than the present situation, where there is virtually no control over who votes or how many times.
What would change is that repeat offenders could be readily identified and treated as such, without the endless fake identities and patchwork of ineffective ID systems that currently exist. When a guy who was convicted of aggravated assault as Joe Smith applies for a job at a nursing home as Tom Brown, claiming to have a clean record, your elderly mother won't get beaten up for wetting her bed, because "Tom Brown" will be quickly identified as the thug he really is, and not get hired to work with vulnerable elderly people (and unscrupulous nursing home operators won't have the "we didn't know" excuse to shield them from liability). And employers, both those who attempt to be law-abiding, and those who've been relying on the "we didn't know" excuse, will have an easy way to identify people who are legally allowed to work here.