Leaving a TV or vid display on when not actually in use ages it unnecessarily. Those are hours of utility value wasted for nothing, and it just brings the deteriorating performance on sooner than later.
For a flat panel LCD set, it's usually the backlighting that starts to go first, the panel next if the power supply caps don't go bad first for both.
For plasma, it's going to be the display panel eventually, if not the power supply.