Importance of Custom Machine Key

I have been launching some new stores, and while monitoring the system warnings, I see an alert for "A custom machine key is not specified (web.config file)" on 2 different 3.4 installs.

Since these are on a single stand-alone server, this shouldn't be needed, correct?

Thanks.

Yes, it's highly recommended. It can prevent your customers from being automatially logged out when application is restarted (for some reasons).