Technical issues aside, a potential downside to using your satellite television receiver's PVR function to record security video - there is a high probability a burglar will inadvertently deny investigators damning evidence of his crime when he steals the PVR.
A cunning invader may make an effort to locate and destroy any security camera recording device on the premises, but his efforts can be frustrated if the recorder is somehow hidden and/or locked away. Your satellite receiver is probably going to be out in the open, next to your television and other expensive consumer electronics that are likely to stolen.