A coat of varnish (what we see a lot of here in the too-****ed-hot Southwest) not only is deleterious to valve stem seals, but to all seals over time as they are not receiving lubrication.
The varnish coat is responsible for internal engine parts not being able to be cooled effectively by circulating oil. A thin film doesn't seem like much, but it can be measured off of the valve stems and springs. They tend to get MIGHTY hot in normal operation (thus the varnish coat).
Auto-Rx sure is great for this. Oil cools not only the bottom end, but is responsible for the valvetrain components not overheating.
Granted, failure may never happen, but why let a potential problem exist that is so easy to fix?