nscd can only be used for caching NSS maps like passwd and groups etc. But it does not handle anything of the PAM stuff.
Also nss-pam-ldapd aka nslcd does not cache password logins coming in through PAM.
But that's exactly what sssd does: It checks the clear-text password against LDAP while in online mode and stores a salted SHA-512 hash to its cache DB. This password hash is used when checking a password while in off-line mode.
Having said this you have to think a bit more about real-world use-cases:
Password caching is always useful when using Linux on a laptop while traveling, that's the perfect use-case for sssd.
If your network goes down you're probably in more general trouble. Now ask yourself:
- Does it make sense trying a login to normal machines? No doubt you have to work somewhere to fix the network problem. But is that your LDAP-integrated Linux system?
- If you have many machines getting redeployed in a data-center, did you ever provide your password on all machines before?
There's one use-case: If for some strange reasons, e.g. a local network driver going crazy, and you have to login through a console (via BMC) to fix this, it would be useful to have a cached password.
But in most of these cases you won't need or can't even use an off-line login.
And if you insist having it you might use a SSH emergency key with proper security controls in place for the private key.
P.S.: And yes, having many LDAP replicas in various racks of your data-center to avoid such an outage is the right thing to do.