Most recent gasoline cars have a direct ignition, also called coil on plug, system where each spark plug has its own ignition coil. Before that, cars used a common ignition coil for all cylinders.
What are the benefits of such a direct ignition system? Is it there for increased reliability or is it actually cheaper to make the high voltage path as short as possible (and thus eliminate distributor and high-voltage cable costs), offsetting the costs of the additional ignition coils needed for such a system?
I recall one incident I had on a 1989 Opel Vectra where the common high-voltage wire from the ignition coil to the distributor was slightly loose and thus was disconnected due to vibration when driving, and the car stopped. Fortunately, I immediately noticed the problem when looking at the engine. Such a failure obviously can't happen on a direct ignition system.
Is it common that such systems require special purpose tools for changing the spark plugs? I at least know that some Volkswagen engines require a special tool for removing the ignition coil so that the spark plug can be accessed below the ignition coil.