The purported wisdom of automatic chain and gear replacements always occurring together, should be challenged by common sense. And anyone telling me that this is what I should do without inspecting my drive train is either lazy, stupid, or both.
Folks that clean their chain and drive system whithout removal and use water based degreasers because they smell good are most likely those that should be replacing their entire drive system every couple years. These might be the same ones that ride their bikes through all types of foul weather and think they're properly maintaining the bike by spraying down with simple green and then hosing it off.
I have observed many bikes that are well cared for that have chainrings and cogsets that have accumulated in excess of 10,000 miles without any skipping or jumping. These bike have gone through numerous chain only replacements. Take the advice of others and frequently inspect and measure your chain wear. If you are taking very good care of your bike, replace the chain every 2000 miles or so and forget about the cogsets and chainrings.