why is actually this necessary? In linux, you could create a PAM module that proofs your identity somehow and then says "ok" to the login subsystem.

There is no need to know your password in order to log you in, if the security subsystem is well designed.

On the other hand, if your home volume is encrypted, the system has to get the right key somehow; so the key itself should be stored in a way that's accessible to the fingerprint auth software (i.e. in clear).

Usually the encryption key is based on the user password but it doesn't have to be the cleartext password. So, using biometrics only, the home volume encryption is weakened, but the password isn't necessarily revealed, and that's an important distinction given that most of the people use the same (or few) password(s) everywhere.