Let's look at it this way...There are 2 ways to view the timing of a pole plant/touch.
Offensively and defensively.
In the traditional "Pianta Su" methodology, it involved a touch of the pole as a balancing mechanism at the point of maximum edge set and discontinuance of the direction change. Most would look at this as occuring at the moment of maximum flexion.
This would be immediately followed by a vertical rising/extension movement, which would then facilitate the edge release and subsequent re-engagement. The fact that the pole is actually interfering with the CM as it tries to transition from one side of the skis to the other, is what drives the CM upwards through that change. It is the only direction it can go, safely.
This is what is viewed as a defensive, or blocking, pole plant. This movement pattern is still valid in certain circumstances, such as steep chutes, or anytime irregular terrain dictates speed control. This type of turn is usually associated with more pivoting actions, than carving actions. This also promoted a very late edge engagement, which usually results in a strong braking, or deccelerating action.
In contemporary skiing, the pole touch occurs after the edge change. Whether the edge change is effected by a vertical movement (combined with a cross over) or by a lateral movement, the touch compliments a fluid movement to the inside of the ensuing turn. It also allows for an earlier edge engagement, resulting in a smoothly carved turn.
This is what is commonly viewed as an offensive, or gliding, poleplant. Even the name "plant" is a misnomer. It exists as a very gentle touch, as the skier slides past the point of contact. It is not used as a primary balancing point, but rather as a simple timing and rythmn mechanism.
I will stipulate that in the WC arena, both are used constantly, as each racer makes adjustments during a run. But from the perspective of the average recreational skier, both are valid tools to have at your command. As you begin to allow your skis to do more of the work for you, you will find it more effortless and enjoyable to use the offensive/gliding timing more often than the defensive/ blocking version.