I would think (yeah, I know, a dangerous thing for me to do) the danger would come when handling the drive before installation or when using internal drives in docks for backups, etc. (like I do; I'm so paranoid, I handle mine while wearing anti-static gloves).
I doubt hdd manufacturer's would consider putting a drive in a dock "proper" installation. The mfr designed the drive to be installed properly by properly trained people that know how to handle the drive. If someone that doesn't know the proper way to handle the drive handles or installs it incorrectly, it isn't the drive mfr's concern.
I've handled hundreds, if not thousands of cards and drives with exposed circuitry over my 30 year career and (to my knowledge) never had one fail because I handled it improperly.
There's no doubt that covering it up would protect it better but it isn't deemed necessary and would drive up the cost needlessly.