If companies encouraged their employees to call in sick when they need to (including for mental health days), employees would probably call in less and be better at their jobs.

It's all about the work environment. Why make someone feel guilty for using their PST? Why make someone feel worse when they need rest to take care of themselves? If encouraging your employees to use days off for their issues and illnesses was a normal thing then people wouldn't do it as much and more than likely they'd be a better employee. Duh.