EMI protection for unused pins
IC's (as a rule of thumb) don't like floating pins but connectors are just totally passive in nature and don't care. 

A floating input pin of an IC can be problematic - not only can it cause logic errors (if part of a logic device) but it can cause analogue errors. This is generally the case when the pin should have been connected to a defined voltage point such as 0V.

However, some chips will have information in their data sheets that tell you not to do anything with certain pins and you have to respect the IC supplier's instructions. I've seen this on a few voltage references.

Connectors don't care but you might decide that to reduce the possibility of EMI susceptibility that unused pins are grounded to the case thus reducing the "aperture" created by a bunch of unconnected pins.