Motorola seems to put these things on their cable boxes in case one of their customers (the cable companies, not the end user) wants to make use of them in the future. It's probably easier to build them directly into the design than it is to try to retrofit them into a design later on.
Each cable company has its own firmware on the boxes, so a Shaw box may look identical to a Comcast box but they might function differently.
Here is Motorola's manual for the 6800 series PVR:
http://broadband.motorola.com/noflas...2659-001-a.pdf
There is no mention of how to use the inputs.