Why do they have to do both? One is simply to secure your voting process against any possible tampering. It is perfectly reasonable to refuse government services and a myriad of other things if one does not have proper documentation. I don't see any reason not to apply the same validation to voter-verification. If you don't do that properly, then you get identity theft and a myriad of other things such as disjoint and variable-quality national databases.

If it was me making these decisions: I'd verify each person against a national fingerprint and facial database before even letting them fill in the ballot form and only accepting their vote if the verification was positive.