Making the conscious decision to start on one mag is necessary only if the airplane doesn't have a START position on the mag switch. If it has the non-start switch (or two toggle switches) and a separate start button, you might need to start on the impulse mag only and switch to both after start. If the engine has two impulse mags there's no need to do that. A mag switch with the start position will cut out the non-impulse mag in the start position.
The impulse coupling does two thing: it retards the spark to near TDC to avoid kickback, and it winds up a spring to snap the mag's rotor forward quickly to generate plenty of electricity for a hot spark.
The Bendix dual mag that Lycoming used on some engines has one drive gear and therefore one impulse coupling. If the coupling fails, and they indeed can, the entire ignition system is in trouble. There was an issue some years ago with the impulse spring corroding and breaking, which allowed the dual mag's timing to fall to near TDC instead of the 25 or 30 degrees before top dead center, and the engine's power would fall to practically nothing. In an airplane with two separate mags the mag that didn't fail would take you home; the dual mag is entirely done when the drive quits.
Continental bought the Bendix magneto line and since their engines didn't use that dual mag, they stopped building it.