I've inherited several board designs where each of the four [plated through] mounting holes are connected to the board's ground plane through a capacitor [1000pf, 1KV]. I can understand why you might want either mounting holes connected directly to signal ground, or why you might want holes that are totally isolated. But I'm confused about the benefit of a connection to ground through the cap.
While metal hardware is used to install the board, there's no guarantee that the material the board is mounted to will actually be in contact with system ground [or will even be electrically conductive].
Background: the boards are part of a piece of commercial food service equipment. All boards have microcontrollers and handle digital I/O, 24V loads. One board has a configuration where it can drive brushed DC motors.