Having root login actually is beneficial to Slackware as it allow an administrator a direct login, while sudo can often be hit or miss with certain executions. By allowing root login, an administrator in administrator mode runlevel 2, can easily diagnose and fix a system, and if the passwords are strong and security is properly maintained, root login can be extremely useful as a tool.
I don't recommend it for day-to-day actions, but there are some things you can only do as root that sudo can't.
As far as PAM... PAM can be compromised and if it is compromised it can render a system completely locked out even to root. PAM is best left alone in my opinion.